Reference for Bava Batra 322:4
מר זוטרא אמר למה לך כולי האי כל מקושר שאין עדיו כלין בשיטה אחת פסול
Any folded [deed the signature of] whose witnesses<span class="x" onmousemove="('comment',' Written vertically across the back of the deed, whether from top to bottom or from bottom to top. ');"><sup>10</sup></span> do not terminate<span class="x" onmousemove="('comment',' On the upper and lower edge of the document. ');"><sup>11</sup></span> with the same line [on the deed],<span class="x" onmousemove="('comment',' I.e., the first letters and last letters of all the signatures must begin and end respectively with the same top and bottom lines of the deed. ');"><sup>12</sup></span> is an invalid [document].<span class="x" onmousemove="('comment',' Hence there is nothing to apprehend. Should one add any spurious matter, it would be detected by the fact that the back of it would protrude below the signatures. Should one cut off a line, the initial or final sections of all the signatures also would thereby be lopped off. ');"><sup>13</sup></span>