Mesechet Gittin 20a 
- The Gemara differentiates between two types or carving: חק תוכות, carving the centers of the letters and their sides, and חק יריכות, carving of the legs of the letters themselves.
There are many letters of the Aleph Bet which contain three walls. The space between these walls is called the “inside” –“תוך” (singular of תוכות). For example, the letters ב,ה,ח,ט,כ,מ,ס and ת clearly have an inside surrounded by three walls. חק תוכות literally means carving the insides of these letters, and implies that the outsides of the letters will be carved out as well in order to form the letter itself. Of course, this rule of חק תוכות applies to all the letters, even part of a letter, and is not limited to letters that actually contain an inside. Mishna Berura,Siman 32,Seif 17
- "תנו רבנן: 'וכתב' - ולא חקק. למימרא דחקיקה לאו כתיבה היא? ורמינהו: עבד שיצא בכתב שעל גבי טבלא ופינקס - יצא לחירות, אבל לא בכתב שעל גבי כיפא ואנדוכתרי! אמר עולא אמר רבי אלעזר: לא קשיא: הא דחק תוכות הא דחק יריכות". The Rabbis taught: [The verse states] “And he wrote” – and not “[and] he carved.” Is this to say that carving is not writing? We have another source that seemingly contradicts this one: A slave who was released by [a documents] writing on a board or notebook – is set free [legitimately]. But [a slave is] not [set free legitimately] by a documents writing on a wool hat or pin. [This contradiction is] not a question: This [new case, which proves that carving is writing,] is carving the centers [of the letters and their sides, and] this [previous case, in which we derived that carving is not writing,] is carving of the legs [of the letters themselves].