does this refer to the size of lines with the space between lines added? Or is it perhaps referring to lines of writing themselves, without their spaces?

Rav Naḥman bar Yitzḥak said: It stands to reason that it is referring to the lines with their spaces. As, if it were to enter your mind that it is referring to the lines without their spaces, for what is one line without its space fit? The baraita did not have to state that a document with a single blank line after the text, measured without counting spaces, is not forgeable; this is obvious. Rather, one may conclude from this claim that the reference is to two lines with their spaces. The Gemara affirms: Conclude from this claim that it is so.

Rabbi Shabbtai says in the name of Ḥizkiyya: With regard to the gap of two blank lines between the text and the signatures, which the Sages said invalidates the document, the lines are measured by the handwriting of witnesses, and not by the handwriting of a scribe, who is presumably skilled enough to write in a smaller script. What is the reason for this? Anyone who forges a document, adding additional lines to the document, would not go to a scribe and ask him to forge it; he would execute the forgery himself, or have another unscrupulous person who is not a professional scribe forge it. Therefore, in order to present a concern for possible forgery, a document must have two blank lines that are measured by the handwriting of an ordinary person, such as one of the witnesses.

The Gemara has established that the width of the gap required to invalidate the document is two lines with interlinear space. The Gemara clarifies: And how much interlinear space is necessary to invalidate the document? Rav Yitzḥak ben Elazar says: For example, enough to write the Hebrew word lekh, and then the Hebrew word lekha, this word on top of that one. These two words each consist of the two letters lamed and final khaf; the former has a projection that fully occupies the interlinear space above it, and the latter has a projection that fully occupies the interlinear space below it. Writing these words one under the other, then, would require an additional interlinear space above and below both lines. The Gemara concludes: Apparently, Rav Yitzḥak ben Elazar maintains that the empty space required to invalidate the document is the width of two written lines with four interlinear spaces.

Rav Ḥiyya bar Ami states a different opinion in the name of Ulla: For example, enough to write a lamed on the upper line and a final khaf on the lower line. The Gemara concludes: Apparently, Ulla maintains that the empty space required to invalidate the document is the width of two written lines with three interlinear spaces, one above the first line, one between the two lines, and one beneath the second line.

Rabbi Abbahu states a different opinion: For example, enough to write the name Barukh ben Levi on one line. Barukh contains a final khaf, and Levi contains a lamed. The Gemara concludes: Apparently, Rabbi Abbahu maintains that the empty space required to invalidate the document is the width of one written line with two interlinear spaces, one above the line and one beneath the line.

§ Rav says: They taught in the baraita that a gap of two lines invalidates the document only if that space is between the witnesses’ signatures and the text of the document. But if there is a gap between the witnesses’ signatures and a court’s ratification of the document, which follows the witnesses’ signatures, then even if there is more space than this, the document is valid.

The Gemara asks: What is different about the case where the gap is between the witnesses and the text, that it invalidates the document? There is a concern that perhaps the holder of the document may forge additional lines and write whatever he wants, and the witnesses have already signed at the bottom, giving the appearance that they attest to the added lines as well. But the same concern can be raised concerning a gap between the witnesses’ signatures and the court’s ratification as well: There, too, he can forge additional lines and write whatever he wants, and have witnesses sign it, with the court’s ratification giving the appearance that it attests to the added lines and signatures as well. Why is this document valid?

The Gemara explains: When is a gap between the witnesses’ signatures and the court’s ratification not problematic, according to Rav? Only when someone inks in the blank space with lines or dots, to prevent information from being added there. The Gemara asks: If so, the gap between the witnesses’ signatures and the text of the document should also be made irrelevant in this manner: Let the scribe ink in [metayyet] the blank space. Why, then, was it taught categorically that the witnesses must sign within two lines of the text?

The Gemara answers: Inking in the gap between the text and the signatures of the witnesses will not help, as people might say, i.e., the concern might be raised: The witnesses are signed only on the inking in. It is possible that the witnesses’ signatures were affixed only to attest that the inking was done in their presence and that the inking in is not a sign of duplicity, and their signatures do not relate to the actual text of the document. The Gemara asks: If so, raise the same concern when the gap between the witnesses’ signatures and the court’s ratification is inked in; there too, people might say: The court’s ratification is signed only for the inking in, and not for the actual text of the document. The Gemara answers: A court does not sign on mere inking in; their ratification is always in reference to the entire document.

The Gemara raises another issue: And let there be a concern that perhaps the holder of the document will excise the entire text that appears above the signatures, and then erase the inked-in part and write whatever he wants in that erased area, and have unscrupulous witnesses sign it. And this would be a valid document, as Rav says: A document that comes before the court for ratification in which its content and the signatures of its witnesses are both written over an erasure is valid. The court ratification would then be assumed to be referring to this new, forged document.