HOW FAR MUST THE TIKUN EXTEND BELOW?
Rejection #1: No, all permit carrying under the Korah;
Rav Yosef holds that a Korah is needed for a Heker, (Therefore, a Tefach suffices.) Abaye holds that a Korah is needed for a Mechitzah. (Pi Tikra Yored v'Sosem applies only up to a height of 20 Amos, and four Tefachim is the Shi'ur that a Mechitzah must enclose.)
Rejection #2: All agree that a Korah is needed for a Heker; Rav Yosef holds that the Heker at the bottom (the area from which people must be able to see the Korah) is like the Heker on top. (A Tefach suffices.) Abaye does not say so. (Therefore he requires an important Shi'ur, i.e. four Tefachim.)
Rejection #3: All agree that the Heker at the bottom is like the Heker on top. Abaye decrees lest the platform be diminished [since people walk on it. Since we need more than a Tefach, we require four Tefachim.] Rav Yosef does not decree.
Question: If a Korah is less than 10 Tefachim above the ground, and one wants to hollow out the floor underneath so it will be 10 above the floor, how much must he hollow out?
Objection: This is obvious! It must be enough so that the Korah will be 10 above it!
Question: Rather, how far must the ditch extend [along the length of the Mavoy]?
Answer #1 (Rav Yosef): It must be four Tefachim.
Answer #2 (Abaye): It must be four Amos.
Suggestion: They argue about the law of R. Ami [and R. Asi]:
(R. Ami and R. Asi): If part of a side wall of a Mavoy near the opening was broken:
If four Tefachim of the wall remain [by the opening], it permits a Pirtzah (breach) up to 10 Amos;
If four Tefachim do not remain, if the Pirtzah is less than three Tefachim, it is permitted [to carry in the Mavoy]. If it is three Tefachim, it is forbidden. (Perhaps people will enter and leave through the Pirtzah for a shortcut, and it will become the primary opening, so the Lechi or Korah by the original opening does not permit.)
Rav Yosef holds like R. Ami. (Since the Korah permits the four Tefachim near it, which is the Shi'ur (minimal length) for a Mavoy, it permits the entire Mavoy). Abaye disagrees. (He holds that the Shi'ur for a Mavoy is four Amos. He requires a remnant of four Amos.)
Rejection: Abaye could agree with R. Ami, who discusses a Mavoy that was once permitted, but a wall was breached. Here, we discuss a Mavoy that was never permitted. Four Amos are required.
Support (Abaye, for himself - Beraisa): A Lechi or Korah permits a Mavoy only if houses [open to Chatzeros,] and [the] Chatzeros are open to the Mavoy.
(The opening of a Chatzer is at least four Tefachim, excluding the posts on the side.) If a Mavoy is only four Tefachim long, there is no room for a Chatzer to open to either side [especially because the side cannot be totally open]!
Suggestion: The Chatzeros open to the middle wall. (I.e. it is more than eight Tefachim.)
Rejection: Rav Nachman cited a tradition that a Lechi or Korah permits a Mavoy only if its length exceeds its width, and houses and Chatzeros are open to the Mavoy.
Answer (for Rav Yosef): The Chatzeros open to the corners [along a diagonal of four Tefachim, somewhat like the diagram in Rashi. However, these openings must span a larger part of the walls than Rashi specifies (Tosfos).]
Support (Abaye, for himself): Rami bar Chama taught that if a protrusion from a side wall extends less than four Amos [along the opening of a Mavoy], it is considered a Lechi, and no other Lechi is needed. If it extends four Amos [and was not built to be a Lechi], it is considered a Mavoy (since this is the Shi'ur Mavoy). Another Lechi is needed.
Rejection (Rav Yosef): If it is four Amos, it is [only a Mavoy, and] not a Lechi. If it is four Tefachim [up to four Amos], it is like a Lechi, and it is also like a Mavoy.
A LECHI THAT IS TOO LONG
(Rami bar Chama citing Rav Huna): If a protrusion from a side wall extends less than four Amos [along the opening of a Mavoy], it is a Lechi, and no other Lechi is needed. If it is four Amos, it is a Mavoy, and another Lechi is needed.
Question: Where do we put the other Lechi?
If it extends past the wall of four Amos, it merely makes it bigger!
Answer #1 (Rav Papa): We put it on the other side.
Answer #2 (Rav Huna brei d'Rav Yehoshua): It can even extend past the wall of four Amos. One makes it taller or lower than the wall, or thinner or thicker [so the extension is Nikar by itself].
(Rav Huna brei d'Rav Yehoshua): This is only in a Mavoy that is [at least] eight Amos wide. If it is only seven [or any amount less than eight] wide, the wall [of four Amos] permits it, for Omed Merubah Al ha'Parutz (the wall, which is four Amos, exceeds the empty space, which is three Amos). We learn from a Kal va'Chomer from a Chatzer:
A Chatzer is not permitted through a Lechi or Korah [in place of one of the walls], yet it is permitted through Omed Merubah Al ha'Parutz. A Mavoy is permitted through a Lechi or Korah, all the more so it is permitted through Omed Merubah Al ha'Parutz!
Question: We cannot learn from a Chatzer. It can have a Pirtzah of 10 Amos, but a Mavoy cannot have a Pirtzah of more than four! (Rashash - the text should read Arba'ah (masculine), for it refers to Tefachim, like the text on 6a.)
Answer: Rav Huna brei d'Rav Yehoshua holds that also a Mavoy can have a Pirtzah of 10 Amos.
Question: Rav Huna brei d'Rav Yehoshua explains Rav Huna's teaching. Rav Huna does not allow a Pirtzah of more than four!
Answer: Rav Huna brei d'Rav Yehoshua teaches his own opinion [about the case that Rav Huna discusses].
Rav Ashi: Even in a Mavoy eight Amos wide, no other Lechi is needed!
If Omed Merubah Al ha'Parutz (the wall is any amount more than four Amos), no Lechi is needed;
If Parutz Merubah Al ha'Omed (the wall is any amount less than four Amos), the wall is a Lechi!
Question: Perhaps the Omed equals the Parutz (i.e. both are exactly four)!
Answer: This a Safek (we did not measure precisely and find that they are equal) about a mid'Rabanan law, so we are lenient.