Reference for Bava Kamma 101:7
ואי ס"ד יש חבטה בפחות מי' למה לי עשרה א"ל שאני בית דכל פחות מעשרה לאו בית הוא
<i>'from thence'</i> but not into it. How is that so? Where the public road was ten handbreadths higher than the roof, and a man might fall from the former on to the latter, there is no liability [in respect of a parapet], but if the public road was ten handbreadths lower than the roof, and a man might fall from the latter on to the former, that there will be liability [in respect of a parapet]. Now, if you assume that a fall could be fatal even from a height of less than ten handbreadths, why should it be necessary to have the public road lower by [full] ten handbreadths?<span class="x" onmousemove="('comment',' Why should there be no liability to construct a parapet even where the public road was lower by less than ten handbreadths. ');"><sup>5</sup></span>