# Techum

This is the approved revision of this page, as well as being the most recent.

A person may not leave the domain where he started Shabbat. This boundary of where he cannot leave is called the techum (heb. תחום; lit. border). Every person has a unique techum depending on where he was at the beginning of Shabbat. It is forbidden to walk beyond one's techum on Shabbat, Yom Tov, or Yom Kippur.[1] In general, the techum boundary is 2000 amot beyond the immediate 4 amot area around a person.[2]

## Determining the Techum

### Outside of a City

1. If a person is alone in the desert, the techum extends 2000 amot beyond the 4 amot around him. If a person is in a house at the beginning of Shabbat, the techum is 2000 amot from outside the house.[3]
2. If a person starts Shabbat outside a city he can determine the directions of his techum and have 2000 amot in each of those directions. He also is allowed to travel until the end of 2800 amot in the direction of the corners of the square he chose. Once he decided his square he cannot change it that Shabbat.[4]
3. If one starts Shabbat in a city, his techum is the entire city. In fact, the city is considered only 4 amot and one may walk beyond that city for 2000 amot. A city is considered like 4 amot whether or not it is a walled city.[5]
4. Additionally, if he starts Shabbat outside a city but his techum entirely includes a city, the city is only considered 4 amot. For example, if one's dwelling place is 500 amot from a city and the city is 1000 amot long, the city is only considered 4 amot. Therefore, one is able to walk another 1496 amot after the city. However, if one's techum ends in the middle of the city, the city is not considered 4 amot and one may not walk beyond one's techum.[6]

### Techum of a City

1. If one is in a village, town, or city, one may have 2000 amot from the outer bounds of the city, depending on the density of the houses.[7] If there are 6 homes each within 70.66 amot of another home, they are considered a city and the techum would begin from 2000 amot outside the group of houses. Any home that is within 70.66 amot of the established city is included in the city. However, a house which is more than 70.66 amot from the other houses is not included in the city. The techum for residents of that house is 2000 amot from the edge of that house. Therefore, a suburban area with houses separated more than 70.66 amot are not considered part of a city and residents of a house only have 2000 amot from that house.[8]
2. Many cities have 2000 amot from the edges of the city. Generaly, the techum is drawn as a rectangle around the outermost extremities of the city along the directions of a compass; that is, the techum of the city is 2000 amot from the edge of the northern most house, eastern most house, southern most house, and western most house. However, if the city is already rectangle, L-shaped, or arc shaped may not have this extension of squaring off the city.[9] See #Squaring Off a City for details.
3. Because the laws of establishing a techum and extending the techum with a Eruv techumin are complicated one should consult a local Orthodox Rabbi. [10]

### Squaring Off a City

1. Chazal learn from the Torah that a city is considered as having corners which are drawn according to the cardinal directions of the world.[11]
2. A city that is a square or rectangle is not squared off because it already has corners, even though its corners do not align with the cardinal directions.[12]
3. A city that is a circle, triangle, pentagon, hexagon, or more sides are squared off.[13]
4. A city with two parallel sides, one side which is wider and one side that is shorter, such as a trapezoid, is squared off by making the shorter side as long as the longer side.[14]
5. A city which has houses within 70.6 amot jutting out on one side, when squaring off the city, the city is measured by the furthest house in that direction.[15]
6. A city that is shaped like an L or a semicircle, if the ends are within 4000 amot, it is all considered one city and the area between its two ends is considered as though it was filled in with houses. If the ends are 4000 amot or more between the ends of the city, each branch of the city are considered separate and the area between them is not considered part of the city.[16] Some are lenient to consider that area part of the city if from the line drawn between the two ends of the city and the middle of the city is less than 2000 amot. Additionally, some are lenient that if the ends gradually spread apart to consider the section of the ends where they are less than 4000 amot apart to be considered filled in.[17]
7. There is a dispute whether a L shaped or triangular shaped city is squared off according to the cardinal directions or by the side that is straight.[18]
8. There is a discussion if it is possible to add onto the square of a city if it goes over a body of water.[19]

### Joining Cities

1. If two cities are within 141.3 amot of each other they are considered like one city and the techum is drawn around both of them.[20]
2. If when drawing the square around a city that would include part of another city, some poskim hold that the two cities are considered one large city and the techum is drawn around both of them.[21]
3. This dispute directly impacts if one may walk from Brooklyn to Manhattan. If one is lenient about the above question, if someone starts Shabbat in Manhattan he can walk to Brooklyn and vice versa. If one is strict about the above dispute he may not.[22]
4. Nontheless, even those who are strict not to consider both cities to be like one large city agree that if someone starts Shabbat within the squaring off of the techum of one city he may walk throughout that city. For example, if he starts Shabbat in a specific section of Manhattan that is within the squaring off of the techum of Brooklyn, he may walk throughout Brooklyn on Shabbat.[23]

## Techum of Specific Cities

### Techum of Manhattan

1. There are two ways to square off the city of Manhattan to determine its techum.
1. Method 1: One way is to draw a rectangle using the western side as a single straight side. A parallel line is drawn with the same length at the same angle to the east of the island. Then two shorter sides are drawn perpendicular to the parallel lines to complete the rectangle. This is Chazon Ish's method to squaring off a city. According to this a small section of Brooklyn and possibly a tiny section of Jersey City are included in the squaring off of Manhattan.
2. Method 2: Another way of squaring off Manhattan is to square it off based on the cardinal directions. The western border is drawn from the western most part of the island, and the same is done for the other directions so that there is a rectangle which aligns with the cardinal directions. This squaring off of Manhattan easily protrudes into Brooklyn as well as Fort Lee.[24]
1. The ramification of this question is whether it is permissible to cross the Williamsburg bridge or Washington bridge on Shabbat. According to the Chazon Ish's squaring off of Manhattan both of these bridges would be beyond the techum; accordingly, it is forbidden to cross these on Shabbat. According to the other method, both of these bridges and some adjacent area is within the squaring off of Manhattan; according to this view it is permissible to cross these bridges on Shabbat.[25]
2. Yet, even according to Chazon Ish's view there are three other arguments that possibly could allow for crossing these bridges. 1) Chazon Ish suggests that cities join together if even just a small part of their squaring off overlap. Rav Elyashiv disagrees.[26] If one were to be lenient on this question, then even according to the first method of squaring off Manhattan it joins with Fort Lee and Brooklyn. 2) Tunnels with buildings on top potentially join cities.[27] 3) The Williamsburg bridge had a booth for guards to watch the bridge. Some poskim hold that this makes the entire bridge like a dwelling place and then it would join Manhattan and Brooklyn together. In practice, some rabbis allow walking over the Williamsburg bridges and others do not.[28]

### Techum of Bronx

1. There are two methods of how to draw the techum of Bronx. Note, the main area of Bronx has contiguous settlement without breaks of 141.3 amot[29] for most of Bronx, Yonkers, New Rochelle, Scarsdale, and White Plains. The map below demonstrates where highways like Bronx River Parkway, I-95, and I-287 break up the settlement.[30] Additionally, parks and rivers break up the settlement.[31] Either way, the settlement does not spill over into Connecticut by measuring contiguous settlement.[32]
1. Method 1: Perhaps Chazon Ish[33] would draw the techum of Bronx as a rectangle beginning with the eastern side which is roughly straight and draw a rectangle from there.
2. Method 2: Bronx isn't a rectangle and so its techum is drawn according to the cardinal directions. Each border is drawn from the furthest point in that direction.[34]

The above maps should not be used for halachic purposes without consulting one's local Orthodox rabbi.

1. Whether it is permitted to walk from Riverdale (Bronx) to Washington Heights (Manhattan) on Shabbat depends on how the techum of Bronx is squared. If Bronx is squared according to the cardinal directions then the 2000 amot techum around that square includes Washington Heights. Accordingly, it is permitted to walk from Riverdale to Washington Heights. However, Manhattan's squaring off (by either method) does not reach most of Riverdale. Accordingly it is forbidden to walk from Washington Heights to Riverdale on Shabbat.[35]
1. Yet, other reasons to permit walking from Washington Heights to Riverdale include: 1) If two overlapping squares is considered one city, then Bronx and Manhattan are considered one city.[36] 2) If the tunnels between Bronx and Manhattan are considered as joining them, then they are considered one city.[37] 3) If someone started Shabbat in a very small sliver of the west side of Washington Heights at the beginning of Shabbat, it is considered as though he was in the square of Bronx. If so, he can walk to Riverdale.[38]

### Techum of Brooklyn

1. The techum of Brooklyn and Queens is squared off according to the cardinal directions.[39] The only question is whether the techum should be a rectangle or follow the curvature of the island near Red Hook, Brooklyn since it is an L shape with a stretch of more than 4000 amot. Either way, the techum of Brooklyn includes most of North Manhattan.
1. Method 1: The city as a whole is measured by cardinal directions and goes as far as Union City, NJ and includes the southern part of Manhattan.[40]
2. Method 2: Only go north from the Queens Midtown Tunnel area because to the southeast there are two inlets in the shape of an L which have a gap of more than 4000 amot.[41]
1. Many poskim hold that Manhattan and Brooklyn are considered one city for purposes of techum.[42] This could be based on several approaches: 1)Chazon Ish suggested that cities join as long as their squared off boxes overlap. This occurs between Manhattan and Brooklyn. However, many poskim including Rav Elyashiv and Rav Dovid Feinstein do not accept this leniency.[43] 2) Minchat Yitzchak[44] writes that the bridges which are built with guard booths join onto the city and connect the cities. Shevet Halevi[45] disagrees with this logic. 3) There are tunnels between Manhattan and Brooklyn and some poskim consider that to join onto the city and connect the cities.[46]
2. According to those who hold that Manhattan and Brooklyn are one city for purposes of techum, it is permitted to walk across the Williamsburg bridge on Shabbat.
3. Staten Island is beyond the techum of Brooklyn.[47]

### Monsey

1. For someone who started Shabbat in Monsey, Manhattan and Westchester are beyond the techum.[48]
2. Good Samaritan Hospital (Suffern, NY) is within the techum of Monsey.[49]
3. Mount Cisco Hospital is within techum of New Square.[50]

### Chicago

1. Skokie and West Rogers Park are within the same techum according to many rabbonim for two reasons: 1) Perhaps a highway doesn't divide a city. 2) The two halves of the city have overlapping squares which according to Chazon Ish's suggestion joins them together.[51]

### Yerushalayim

1. Machaneh Yisrael[52] discusses the techum of Yerushalayim at great length. In the north, he suggests that Ramat Shlomo is the northmost border and Ramot is excluded. To the west there's roughly a border from Gavat Shaul to Har Nof to Hadasah Ein Karem. To the south there is a border from Ganim to Gila. Generally, there is no very large squaring off of the city in his opinion because there are several sections which are in the shape of a bow or an L that are filled in. Once that section is already filled in, another larger square is not drawn around it.
2. Another option he raises is to draw the squaring off of the city based on the Rabbanut Eruv map. He is hesitant to do so because perhaps Ramot cannot be joined to the rest of the city.
• Do not rely on this map at all for practice as it is drawn imprecisely just to give a very rough sense of the main shape of the city of Yerushalayim.

## Moving Packages Received on Shabbat

1. If one isn't sure if an item was brought from beyond the techum on Shabbos, one may not to move it beyond 4 amot even though techum is rabbinic.[53]
2. Even if the package was delivered from outside of the techum if the package was placed in a house or building then it can be moved within the house since the house walls are considered 4 amot.[54]
3. If a package was delivered from outside the techum on Shabbat for a Jew he cannot benefit from it on Shabbat or even after Shabbat the time it would take to bring it from beyond the techum. He is allowed to move the package up within 4 amot or if it is in a building within the walls of the building.[55]

## Walking to the edge of the techum

1. It is forbidden to walk to the edge of the techum in order to leave on a journey quickly after Shabbat. However, if the action one is going to do after Shabbat could theoretically have been done on Shabbat, it is not forbidden to walk tot the edge of the techum waiting for the end of Shabbat. For example, one may walk to the edge of the techum in order to bring one's animal back because theoretically one could have done this on Shabbat if there were houses extending the techum. Also one may walk to the edge of the techum in order to collect fruit which fell before Shabbat and aren't Muktzeh because theoretically one could have done so on Shabbat if there were walls surrounding the path (which would permit carrying on Shabbat). [56]
2. One may walk to one's garden within the techum in order to pick fruit after Shabbat since it is not evident that one is walking there for that purpose.[57]

## Techum above Ten Tefachim

1. There is an unresolved discussion in the Gemara whether there is techum above ten tefachim. We are lenient if a person started Shabbat above ten tefachim that he doesn't have a techum of two thousand amot as long as he didn't yet land or reach a place that is connected to the ground.[58] This is relevant to a person who was on a boat at the beginning of Shabbat.[59]
2. A wagon is considered within ten tefachim of the ground since the wheels rest on the ground and the wagon itself if bigger than 4 tefachim.[60]

## Techum of Property

1. A barrel that belongs to two people that was split up on Yom Tov, each part has the techum of the owner of that half even though it was only split up on Yom Tov.[61]

## Boats

1. Some permit getting on a boat before Shabbat even though the boat will travel more than the techum on Shabbat because the traveler isn't walking or moving himself, rather the water is moving the boat.[62] However, most rishonim disagree and hold that traveling more than the techum on a boat is forbidden.[63] Nonetheless, there are other leniencies about traveling beyond the techum on a boat:
1. If the boat is always above 10 tefachim from the seabed there is no prohibition to go in the boat beyond techum since there is no techum above 10 tefachim in the water, according to many poskim.[64]
2. Some permit getting on a boat more than 3 days before Shabbat even though it is known that it will continue to travel on Shabbat more than the techum, even if it is within 10 tefachim.[65]
3. Nonetheless, it is forbidden to get on a boat on Shabbat itself.[66]
4. If someone was on a boat from before Shabbat and it was always above 10 tefachim, he may get off the boat. Once he gets off the boat he only can walk 2000 amot. Some are strict to hold that he may not get off the boat.[67]

## Airplane that Landed on Shabbat

1. It is forbidden to go on a plane that will land on Shabbat.[68]
2. If someone was in an airplane that landed on Shabbat, because of circumstances beyond his control, there is a discussion if it is permitted to get off the plane because of techum. Some allow getting off the plane into the walkway that is attached to the plane and airport. Still one may not leave the airport unless it is included in the techum of the city.[69]

## Someone who came from beyond the techum

1. Someone who was brought against his will from beyond the techum and was brought against his will back into a walled area that area is considered like 4 amot.[70]

## Who Needs An Eruv Techumin?

1. A child up to the age of 7 follows his mother's techum.[71]

1. See a summary of the halacha's of techum with pictures on techumShabbos.com
2. פניני הלכה, שבת פרק ל

## Sources

1. Shulchan Aruch O.C. 396:1, 495:1, 611:1. Peninei Halacha discusses why the allowance for ochel nefesh doesn't permit going beyond the techum on Yom Tov.
2. Shulchan Aruch O.C. 397:1, Kitzur Shulchan Aruch 95:1. Mishna Brurah 397:1 writes that some hold that walking beyond 12 mil on shabbat is a biblical prohibition, while others consider it a rabbinic prohibition. Magen Avraham 404:1 quotes a dispute between the Maharalbach 28 and Maharam Elshaker 41 whether 12 mil is deoritta for kelim. Ramban Eruvin 43a clearly hold it is deoritta even for kelim. Biur Halacha 404:1 is lenient to rely on Maharalbach since either way many rishonim hold that 12 mil is not deoritta.
3. 39 Melachos (Rabbi Ribiat, vol 4, pg 1386-7)
4. Chazon Ish 110:24 based on Gra 345:1, Mishna Brurah 345:5. This is the opinion of the Rambam. According to the Rabbenu Tam (Eruvin 51a, Yoma 67a a person always can travel until 2800 amot even without choosing the square. However, Rashbam held that one's square is automatically determined to correspond with the north-south directions of the earth and then one can walk until the end of those corners. Tosfot Yoma and Rosh Eruvin reject Rabbenu Tam.
5. Tur 398:9, Bet Meir 398:1, Chayey Adam 76:11, Shaar Hatziyun 408:13
6. Kitzur Shulchan Aruch 95:12
7. Kitzur Shulchan Aruch 95:2
8. 39 Melachos (Rabbi Ribiat, vol 4, pg 1387-9). Rabbi Ribiat quotes the Minchat Shabbat who explicitly writes that we do have the halachic category of chatzerot nowadays in order to form a city. We still require that there's 2 houses to create a chetzer but if there's 6 houses we have a city. Torat Shlomo Eruvin 19:19 p. 144 explicitly writes that even though we don't use the chatzerot today like rooms you can create a city without chatzerot. See Chazon Ish OC 110:20 who supports this. However, Rav Hershel Schachter (Shabbat Shiur 99 (very end) and Shabbat Shiur 100 (beginning)) holds that nowadays we don't have a city for techum since we don't use our chatzerot today like a room of the house. Therefore, since we need 3 chatzerot to form a city (Shulchan Aruch 398:10) we can't have a city today. This is based on the Rama, Chazon Ish OC 65:52, and is discussed by Imrei Baruch Eruvin 40-41.
9. 39 Melachos (Rabbi Ribiat, vol 4, pg 1391-2)
10. 39 Melachos (Rabbi Ribiat, vol 4, pg 1393)
11. Eruvin 55a, Shulchan Aruch O.C. 398:3
12. Eruvin 55a, Shulchan Aruch O.C. 398:1
13. Eruvin 55a, Shulchan Aruch O.C. 398:2
14. Eruvin 55a, Shulchan Aruch O.C. 398:4
15. Eruvin 55a, Shulchan Aruch O.C. 398:6
16. Eruvin 55a, Shulchan Aruch O.C. 398:4
17. Rama 398:4
18. Shulchan Aruch Harav 398:3 and Chayei Adam 68:14 hold that a city that is a triangle or L shaped city is squared off by the cardinal directions. However, Chazon Ish 80 s.v. tos 54a and 110:23 maintains that a city which has one side that is straight is squared off in accordance with that side and not according to the cardinal directions.
19. Rav Yitzchak Shpitzer and Rav Yechezkel Shraga Weiss (Poalim LTorah v. 17 p. 118) writes that although some question the fact that a squaring of a city can include an area over water, from Chatom Sofer 94 it is evident that it can include an area over a body of water. Mechazeh Eliyahu 1:74 also addresses this point and concludes that the squaring off of a city can go over water.
20. Shulchan Aruch O.C. 398:7
21. Minchat Yitzchak 8:33 explains that since the squaring of a city is taken into account before we add the 70 amot of the city if the squaring off of a city includes another city everything should be considered one large city. His proof is the concept of Eruvin 55a that the arms of a bow-shaped city join as long as they are within 4000 amot of each other. Kovetz Chaburot Halacha v. 3 p. 26 questions his proof. The Chazon Ish 110:16 wasn't sure whether or not the cities join when the squaring off of each other join. Rav Chaim Kanievsky in Shoneh Halachot 398:19 and 21 writes that Chazon Ish held that they do join but left it unresolved (הדבר צריך הכרע). Zecher Tzvi (techum Shabbat p. 21) holds that one can be lenient based on Chazon Ish. Dirshu 398:21 quotes Rav Dovid Landau who holds that since Chazon Ish left this unresolved one must be strict. Dirshu 398:21 cites Rav Wosner (Kitzur Hilchot Medidat techumin) as holding that one can be lenient, but Rav Elyashiv as holding that one should be strict. Mechzeh Eliyahu 1:74 and 77 is lenient. Machneh Yisrael of Rabbi Dimitrovsky p. 19-22 writes that one who is lenient has what to rely upon and supports this from the Rambam and Meiri. He also cites Rav Elyashiv as being strict. Mdarkei Hatechum p. 18 quotes Rav Dovid Feinstein, Rav Belsky, Rav Elyashiv, and Rav Nissim Karelitz as holding that we cannot join cities based on the squaring off of the city. Gvul Rishonim p. 78 quotes Rav Shmuel Kamenetsky, Rav Yakov Bloi, Rav Yakov Kamenetsky, and Rav Moshe Feinstein held that cities can be joined by squaring off each one. He also writes that he heard from reliable sources that Rav Elyashiv was lenient in a case of need but knows that Machaneh Yisrael quotes Rav Elyashiv as being strict. He also quotes that he heard Rav Dovid Feinstein was lenient, even though others quote him as being strict.
22. Mdarkei Hatechum p. 19
23. Mdarkei Hatechum p. 19
24. Rav Yitzchak Shpitzer and Rav Yechezkel Shraga Weiss (Poalim Ltorah v. 17 pp. 113-131)
25. Rav Yitzchak Shpitzer and Rav Yechezkel Shraga Weiss (Poalim Ltorah v. 17 pp. 113-131)
26. See #Joining_Cities above.
27. Kovetz Chaburot Halacha v. 3 p. 36 notes that the discussion of a subway tunnel joining cities rests upon assuming like Mishna Brurah that if there's a bet dirah in part of the tunnel the entire thing joins onto the city. However, according to Chazon Ish 110:28 argues that only the bet dirah joins and not the rest of the tunnel, there is no basis for this argument.
28. Rav Yitzchak Shpitzer and Rav Yechezkel Shraga Weiss (Poalim Ltorah v. 17 pp. 113-131). Rav Hershel Schachter (Eruvin 118, min 50-53) also notes that whether Manhattan and Brooklyn join together for one techum depends on whether two overlapping techumin are considered one city.
29. Shulchan Aruch O.C. 398:7 writes that two cities join together if they're within 141.3 amot. This is based on Rav Huna in Eruvin 57a and accepted by the poskim.
30. Mdarkei Hatechum p. 10 quotes a debate among the rabbis of Chicago whether a highway breaks up a city for purposes of techum if the settlement on either side is further than 141.3 amot apart. Some rabbis including Rav Moshe Heinemann hold certainly a highway does not break up the city since it is meant to be part of the city and others disagree. Mdarkei Hatechum concludes that it does break up the city. Techum Shabbat Umedidato p. 34 holds that highways do not break up a city for purposes of techum.
31. Shulchan Aruch O.C. 398:9 writes that a valley that is usually dry joins onto the city if it is used by the inhabitants of the city. Mishna Brurah 398:46 concludes that other types of areas do not join onto the city just because they are used by the townspeople. Therefore, it is clear from Mishna Brurah that a river that is always flowing does not join onto a city. This is also the assumption of many poskim who discussed cities with rivers splitting them up including: Meishiv Dvar (4:58 s.v. ach, cited by Dirshu 398:63), Mechzeh Eliyahu 1:74, and Minchas Yitzchak 8:33.
32. See Kovetz Chaburot Halacha v. 3 p. 32 which writes that using Google maps to measure distances between houses is easy and accurate according to the halacha.
33. 110:23
34. Rav Hershel Schachter (Eruvin 118, min 53-56) assumes Bronx is squared off by the cardinal directions.
35. Rav Hershel Schachter (Eruvin 118, min 53-56)
36. Rav Hershel Schachter (Eruvin 118, min 50-56)
37. See Rav Yitzchak Shpitzer and Rav Yechezkel Shraga Weiss (Poalim Ltorah v. 17 pp. 113-131) and Tikkun Eruvin (fnt. 89).
38. See #Joining_Cities above.
39. Kovetz Chaburot Halacha v. 3 p. 36 writes that since Brooklyn and Queens have contiguous settlement they are one city. He documents how to cross Prospect Parkway, Ocean Blvd, etc. On this basis he writes that Brooklyn and Queens' shape is not rectangular and has no straight side. Therefore, even Chazon Ish agrees that the techum of Brooklyn should be according to the cardinal directions.
40. Gvul Binyamin p. 302. Kovetz Chabuot Halacha v. 3 pp. 32-33 discusses this as well. See next note.
41. Kovetz Chabuot Halacha v. 3 pp. 32-33. The reason to follow the cardinal directions for Brooklyn is based on Mechzeh Eliyahu 1:82's understanding of Chazon Ish 110:26 that a L shaped area which has an angle smaller than 90 is not judged like an L. Instead, it is judged by the cardinal directions. However, Zecher Tzvi (Techum Shabbat Umedidato p. 11) argues that Chazon Ish meant it isn't judged like an L and is squared off as long as the gap isn't greater than 4000 amot. According to this approach, the techum of Brooklyn does not start with the furthest point to the west. Rather, it only starts at about the Queens Midtown Tunnel since from there going northeast the east side of the island is pretty straight. Rav Moshe in Igrot Moshe OC 4:88 implies that the squaring off of Brooklyn begins from the southern part of Brooklyn and includes the Lower East Side of Manhattan. This implication isn't clear; see Gvul Binyamin p. 302 for lengthy discussion of that point.
42. Minchat Yitzchak 7:24 quotes the Satmer Rebbe held that Manhattan and Brooklyn were one city for purposes of techum. Rav Yechezkel Roth (Emek Hahalacha 3:25), and Otzrot Halacha (v. 6 p. 427) agree. See Mishna Halachot 8:178. However, Gvul Binyamin (p. 298) quotes Rav Yishayhu Shimonovitz in the name of his father and Rav Yaakov Kamenetsky that in fact it is forbidden to cross the Williamsburg bridge on Shabbat. Gvul Binyamin argues that the minhag to walk over the Williamsburg bridge is not because of joining cities with overlapping squares but because the squaring off of Brooklyn includes most of Manhattan and the squaring off of Manhattan includes Williamsburg. Walking to the end of the squaring off the city one started Shabbat in is certainly permitted.
43. See #Joining_Cities above.
44. 7:24
45. 4:40
46. Rav Yitzchak Shpitzer and Rav Yechezkel Shraga Weiss (Poalim Ltorah v. 17 pp. 113-131) and Tikkun Eruvin (fnt. 89)
47. Otzrot Halacha (v. 6 p. 427)
48. Otzrot Halacha (v. 6 p. 427)
49. Otzrot Halacha (v. 6 p. 427)
50. Otzrot Halacha (v. 6 p. 427)
51. Mdarkei Hatechum (Rabbi Mordechai Melanchik)
52. pp. 73-91
53. Beitzah 24b, Shulchan Aruch O.C. 515:1
54. Shulchan Aruch O.C. 405:1 writes that people who leave the techum only have 4 amot. Shulchan Aruch 405:9 applies these laws to objects as well. Shulchan Aruch 405:6 writes that if a person left the techum accidentally or was forcibly was moved by non-Jews and ended up beyond the techum within a walled area the entire walled area is considered like 4 amot. However, if a Jew left the techum intentionally he only has 4 amot. However, with respect to objects that left the techum, Mishna Brurah 405:50 writes that if the objects left the techum and entered a walled area they can be moved within the entire walled area whether they were moved there intentionally or unintentionally.
55. Shulchan Aruch O.C. 325:8. Shulchan Aruch quotes two opinions about whether he must wait until the time it takes to bring it from beyond the techum in order to eat it. Mishna Brurah 325:40 implies that he is strict about this question, though he doesn't require waiting until Sunday morning the time it takes to deliver it from beyond the techum like the opinion Rama quotes.
56. Kitzur Shulchan Aruch 90:3
57. Kitzur Shulchan Aruch 90:3
58. Gemara Eruvin 43a-b, Shulchan Aruch OC 404:1
59. Shulchan Aruch 404:1
60. Shoel Umeishiv 5:3
61. The Gemara Beitzah 37b records a dispute between Rav and Shmuel whether we hold of the concept of berierah. Berierah is that we can view an eventual decision as though it already happened to clarify what is the case now. Rav holds of it and Shmuel does not. The gemara’s conclusion (38a) is that for derabbanan concepts we hold of Berierah. This is codified by the Rambam (Yom Tov 5:20) and Shulchan Aruch 397:10.
62. Rashbam cited by Tosfot Eruvin 43a, Ramban Eruvin 43a, Rashba Eruvin 43a
63. Rabbenu Chananel Shabbat 19a, Rambam (teshuva 308-310), Ritva Eruvin 43a, Rif Shabbat 7b according to Bet Yosef 248. Maharik 45 understands that Rif can hold like Rashbam. Chazon Ovadia v. 1 pp. 127-130 proves that the poskim don't accept the Rashbam and that's how he holds.
64. Rabbenu Chananel Shabbat 19a, Rambam (teshuva 308-310), Rif Shabbat 7b, Shulchan Aruch 248:2
65. Maharik 45, Rama 248:1, Shulchan Aruch Harav 248:3
66. Mishna Brurah 248:21
67. Gemara Eruvin 43a has a unresolved question about whether there is techumin above 10 tefachim. Rambam (teshuva 308) and Ramban (Eruvin 43a) are lenient if the question is only rabbinic. Rosh, however, is strict that there is techumin above 10 tefachim. According to Rambam and Ramban, it is permissible to get off the boat if he was on there from before Shabbat since his techum didn't begin while he was on the boat above 10 tefachim. According to Rosh he may not get off the boat. Shulchan Aruch 404:1 holds like Rambam and Ramban. Mishna Brurah 248:22 writes that it is permissible to disembark from the boat and go up to 2000 amot if he was on the boat from before Shabbat.
68. Igrot Moshe OC 3:97
69. Igrot Moshe OC 3:97 holds that even if there’s no techum while the airplane is moving, there is techum once it lands. Since from the point it landed until the terminal is more than 2000 amot, it is forbidden to get off the plane. He's discussing leaving from the plane onto the tarmac. techumei Shabbat p. 48 is lenient in extenuating circumstances to get off the plane into the walkway attached to the plane since it is all like 4 amot. See also Tzitz Eliezer 1:21:18.
• Rosh (perek mi shehotziyhu 10) holds that in order for the city to count like 4 amot for techumin it must have walls made for living. This is true both when acquiring techum at the beginning of Shabbat and if brought from beyond the techum and put into walls.
• Rabbenu Yerucham (cited by Beit Yosef 405:6) holds that in order for the city to count like 4 amot it must have walls made for living only when brought from beyond the techum. However, when acquiring techum at the beginning of Shabbat walls aren't necessary. Biur Halacha (405:8 s.v. im) rules in according with this view. Dirshu 405:17 also infers this from Shaar Hatziyun 408:13.
• Ramban (Eruvin 41a) holds that walls are never necessary for a city to be considered 4 amot. Biur Halacha (405:6 s.v. mukfet) writes that in an extenuating circumstance you can rely on this opinion, unlike Shulchan Aruch O.C. 405:6 who rules unlike Ramban.
70. Ketubot 65b, Shulchan Aruch O.C. 414:2