Bava Batra 161B

לאו דיעקב הוא.

is not Yaakov’s signature. That is, it is known for a fact that there is no one living in the city where the document was written who is named Ya’akov and whose signature matches the signature on this document. Therefore, the court will recognize that the signature must have originally stated: “So-and-so, son of Ya’akov, witness,” and that the last line had been excised, and they will invalidate it.

ודלמא בשמיה דאבוה חתם! לא שביק איניש שמיה דידיה, וחתים בשמיה דאבוה.

Rami bar Ḥama challenges: But perhaps this witness signed using the name of his father instead of his own name, as a gesture of respect toward his father. The Talmud answers: This is not done; a person does not discard his own name and sign using only the name of his father.

ודלמא סימנא בעלמא הוא דשוויה! דהא רב צייר כוורא, רבי חנינא צייר חרותא, רב חסדא סמך, רב הושעיא ⁠– עין, רבא בר רב הונא ⁠– מכותא! לא חציף אינש לשוויה לשמיה דאבוה סימנא.

Rami bar Ḥama challenges further: But perhaps the witness made this name into a mere distinguishing mark that he uses as his signature, as it is known that Rav used to draw a fish as his signature mark, rather than signing his name, and R' Ḥanina used to draw a palm branch as his signature mark, and Rav Ḥisda used to sign just the letter samekh, and Rav Hoshaya used to sign just the letter ayin, and Rava bar Rav Huna used to sign his name by drawing a ship’s mast [makhota]. The Talmud answers: A person is not so insolent as to use his father’s name as a distinguishing mark.

מר זוטרא אמר: למה לך כולי האי? כל מקושר שאין עדיו כלין בשיטה אחת ⁠– פסול.

Mar Zutra said: Why do you need all this? Why go to such lengths to answer the question posed above? There is a simpler answer: Any tied document whose witnesses do not end on a single line is not valid. The Talmud had previously assumed that the witnesses sign one after the other, beginning at the document’s bottom line and going upward toward the first line; this arrangement leaves open the possibility that the signature of the first witness could be truncated by an unscrupulous party. Mar Zutra explains that this is not so; rather, the signatures are written with each one beginning opposite the bottom line and heading upward toward the beginning of the document. Therefore, if a line of text is excised from the bottom of the document, the names of all the witnesses on the reverse side will be truncated, and the forgery will become apparent.