Reference for Bava Batra 5:21
אמר אביי שמע מינה כל ביני אורבי טפח הני מילי בטינא אבל בריכסא בעי טפי ואיכא דאמרי הני מילי בריכסא אבל בטינא לא בעי כולי האי
is to allow for the projection of the rough edges? May it not be that gebil is half the thickness of gazith, and this extra handbreadth is to allow for the mortar between the rows, in the same way as we defined kefisin to be half-bricks and lebenim whole bricks, the extra handbreadth being for the mortar between the rows? — He replied: Granting your analogy [between gebil and kefisin], how do we know that kefisin means half-bricks? From tradition. Similarly we know from tradition [that gebil means untrimmed stones]. According to another version, R. Aha the son of R. Awia said to R. Ashi: How do we know that kefisin means half bricks and the extra handbreadth is for the mortar between the rows? May it not be that kefisin means untrimmed stones and the extra handbreadth is for the projection of the rough edges, in the same way as we define gebil to be untrimmed stones and gazith to be polished stone, the extra handbreadth being for the mortar between the rows? — He replied: Granting your analogy [between kefisin and gebil], how do we know that gebil means untrimmed stones? From tradition. So we know this also from tradition.