1) WHAT IS CONSIDERED TWO LINES?
OPINIONS: The Gemara earlier quotes a Beraisa which states that if there is a space of two lines between the body of the Shtar and the signatures of the witnesses, the Shtar is invalid. The Chachamim decreed that such a Shtar is invalid because it provides an opportunity for forgeries. Although no information may be derived from the last line of the Shtar, the second-to-last line may be used to forge details which were not part of the original deal. The Chachamim therefore decreed that such a Shtar (even if a blank space is left and no forgery is present) is invalid.
How much space constitutes two lines? Rav Nachman states that it must be the height of two lines plus the amount of space that is normally left because of each line. Rebbi Shabsi states that the two lines is judged according to the larger handwriting of the owner of the Shtar, and not according to the handwriting of the Sofer, since a Sofer usually does not agree to forge a document. How much is that space? Rav Yitzchak says that there must be a space for two lines of writing plus four additional spaces, one space above and one space below each of the two lines. Since the witnesses are not accustomed to writing a Shtar in such a way that the letters which extend downward (such as the Chaf Sofis) and the letters that extend upward (such as the Lamed) should not collide, they will make large spaces for each letter to have its own area. This results in two lines and four spaces.
Rav Chiya says that although the forger will need a space above the top line (for a Lamed) and below the bottom line (for a Chaf Sofis), he does not need two spaces between the two lines. One space is enough, since he is able to arrange any Lamed in the lower line in a way that it will not collide with the Chaf Sofis of the preceding line.
Rebbi Avahu expresses a third opinion. He understands that when the Beraisa says "two lines," it refers to the spaces above and below one actual line of writing.
Which of these opinions does the Halachah follow?
(a) The RAMBAM (Hilchos Malveh v'Loveh 27:4) writes that the amount of two lines is two lines plus their spaces, such as when a Lamed is on top of a Chaf Sofis. This implies three spaces, because if the Rambam meant four spaces he would have used the example of the word "Lecha" over the word "Lecha," which would take up four spaces (like the opinion of Rav Yitzchak in the Gemara). Which opinion does the Rambam follow? The MAGID MISHNEH explains that the Rambam rules like Rebbi Shabsi. Apparently, the Rambam prefers to rule like Rav Nachman who says that the two lines are "Hen v'Aviran" -- "they and their spaces." Since the natural spaces would be only one space above and one space below each line, with one space between every two lines, Rav Nachman must mean that only three spaces are necessary. The Rambam therefore rules like Rav Nachman and Rav Shabsi. This is also the understanding of the ROSH, who says that this opinion seems to make the most sense. The Magid Mishneh also quotes the RA'AVAD and RABEINU YONAH who side with this opinion.
The BEIS YOSEF (CM 45:9) takes issue with the Magid Mishneh's understanding of the ruling of the Rambam. Everyone agrees with Rav Nachman that the two lines include the space needed to write letters that descend or extend upwards. The only question is how much space is necessary for those letters. The Rambam therefore does not rule like Rav Nachman, since Rav Nachman does not have a specific opinion in this argument.
The LECHEM MISHNEH defends the Magid Mishneh's explanation. He writes that Rav Nachman definitely does not agree with Rebbi Avahu, who says that the "lines" in the Beraisa refer to spaces. This is clear from Rav Nachman's statement, "they and their lines." The Magid Mishneh's reasoning is that this is why the Rambam does not rule like Rebbi Avahu here, even though normally he would rule like Rebbi Avahu since he is the later Amora. Since Rav Nachman can follow only one of two opinions in the Gemara, the Rambam clearly can rule only like the opinion of three spaces and not four, because Rav Chiya is a later Amora, and also because his opinion takes the middle ground. (Interestingly, the Lechem Mishneh does not give the Rosh's reason that this opinion is simply more logical.)
(b) RAV HAI GA'ON, the RAMBAN, and the BA'AL HA'ITUR indeed rule like Rebbi Avahu. The only reason the Ramban gives for why he rules like Rebbi Avahu is that we are stringent with the owner of the Shtar. However, according to the answer of the Lechem Mishneh mentioned above, another reason why the Ramban rules like Rebbi Avahu may be that Rebbi Avahu was the later Amora. (Y. MONTROSE)
2) HALACHAH: THE SOURCE FOR THE CUSTOM TO DESIGN A KESUVAH
The Gemara teaches that a Shtar in which the body of the Shtar is written on the same line as the signatures of the witnesses is valid. TOSFOS (DH Shitah) states that for this reason, a witness should always sign at the beginning of a line and not leave any empty space for someone to insert additional information at the beginning of that line. Since such information would be considered valid, signing in the middle of a line or at the end of a line would create the opportunity for a forgery.
This Halachah has a practical application in a common custom observed today. The TASHBATZ (1:6) writes that a Kesuvah was originally written in an aesthetically pleasing way by leaving wide margins. However, this created the opportunity for an unscrupulous person to fill in more information in the margins. It therefore became the custom to fill in the margins with pictures and designs, to ensure that no additional information would be added. However, the Tashbatz writes, one should make designs or write poetic lines in the margins of the Kesuvah rather than actual verses, due to the Halachic problems involved with writing verses, which he discusses at length in an earlier Teshuvah (1:2).
The Tashbatz endorses the custom of decorated the Kesuvah, and he writes that he personally arranged for the Kesuvos of his sons and daughters to have designs and poetic phrases in the margins. (Y. MONTROSE)