- Twin korahs
The Mishnah on Daf 13b stated that a korah must be wide enough to hold an אריח - a half-brick. An אריח is one and a half tefachim, which means that it is sufficient for the korah to be one tefach in width.
In a Baraisa on this Daf it was taught, שתי קורות המתאימות - if one placed two twin korahs over an entrance, and each one alone was not wide enough to hold the ariyach, but together they could, since combined they are a tefach wide, no other korah is necessary. If their combined width was not a tefach, they are invalid and another korah must be used. Rabban Shimon ben Gamliel says that if the two korahs can be positioned to hold an ariyach lengthwise, so that the distance between their outer edges measures three tefachim, it is not necessary to bring another korah, even though their combined width does not equal a tefach.
- Determining the width of a round korah
The Mishnah on Daf 13b also stated that if the korah was round, and therefore incapable of holding an ariyach, we view it as if it was square, meaning as if it was plane down to a flat surface. The way to know if the circular korah is a tefach wide, is by following the rule thatכל שיש בהיקפו שלשה טפחים יש בו רוחב טפח – Whatever round object has a circumference of three tefachim has a width of a tefach.
The Gemara teaches that the source for this principle is learned from the ים של שלמה, which Shlomo Hamelech made in the Beis Hamikdash for the Kohanim to use for tevillah. It was ten amos in diameter and thirty amos in circumference. Rebbe Chiyah taught that it could hold enough water for one hundred and fifty mikvaos.
- A lechi in the middle of the mavoi
A Baraisa taught, עשה לחי לחצי מבוי אין לו אלא חצי מבוי – if one made a lechi for half a mavoi (so that the lechi stands in the middle of the side wall instead of the front), he can only carry in the mavoi half that was adjusted.
After questioning why the Baraisa needed to teach this seemingly obvious point, the Gemara answers that one might have thought that it should be prohibited to carry even in the adjusted half, out of concern that someone will think that it is permitted to carry in the entire mavoi.
