Question: At what distance will the Sechach above invalidate the Sechach below?

Answer (R. Huna): A Tefach (less than a Tefach does not create an Ohel for Tumas Mes, as we learn in the Mishnah).

Answer (R. Chisda and Rabah b. R. Huna): Four Tefachim (since the minimum dimension for an independent domain is four Tefachim).

Answer (Shmuel): 10 Tefachim.

Question: What would be his rationale?

Answer: The Pesul of a Sukah above should mirror its dimension for being Kosher (height of 10 Tefachim).

Question: But the Rabanan who argue with R. Yehudah hold that the lower Sukah would be Pesulah even if the upper were less than 10 Tefachim in height!?

R. Yehudah taught in our Mishnah that if the upper Sukah does not have residents, the lower Sukah is Kesheirah (by which we infer that the Rabanan would hold that it is Pesulah).

What is that case?

It cannot be simply that nobody is living in the Sukah above, since the Pesul cold not be dependant on the presence or absence of residents in the upper Sukah!

It must be that the upper Sukah is unfit (which permits the lower Sukah), and its lack of fitness must be due to its being shorter than 10 Tefachim.

Then Rabanan, who prohibit the lower Sukah in such a case, must hold that the upper Sukah invalidates even if it is less than 10 Tefachim in height!

Answer (R. Dimi): The lack of fitness spoken of is the floor of the upper (which is the Sechach of the lower) being unfit for sleeping/eating upon.

Question: Then we must infer that the Rabanan would hold that such an unfit upper Sukah would still invalidate the lower Sukah (and it still poses a question on Shmuel)!

Answer: The argument between R. Yehudah and the Rabanan is not over a Sukah whose upper level cannot be used.

All would agree that the lower Sukah would then be valid.

Rather, the argument is whether an upper Sukah which could be used, but with considerable difficulty, is considered a Sukah to invalidate the lower one (Rabanan) or not (R. Yehudah).

If a sheet were spread over Kosher Sechach to supplement its shade or under the Sechach to catch thistles which might drop from the Sechach, the Sukah is thus invalidated.

If such a decorative sheet were spread for decoration over the four posts of a bed, the person under such a spread is not considered as being in the Sukah.

If, however, the bed had two posts, one at the head and one at the foot, and the sheet hung at an angle like a tent from the bar (less than a Tefach wide) connecting the two posts, it is Kesheirah.

(R. Chisda): The Pesul of the spread sheet is only if it is intended to catch thistles, but if it is intended to beautify the Sukah, it is Kesheirah.

Question: That is obvious, as the Mishnah explicitly states that the sheet was spread to catch droppings!?

Answer: I might have forbidden any sheet, and attributed the Mishnah's reference to droppings as using a common example.

Question: The Beraisa seems to support R. Chisda's teaching.

The Beraisa is discussing deriving benefit from decorations hung from the Sukah, with or without a condition made at the time they were hung.

Among those adornments is a colorful sheet, and the Sukah is Kesheirah!

That is not a support, since the sheet in the Beraisa could be understood to have been hung vertically (but be Pasul if hung under the Sechach) and R. Chisda permits the decorative covering under the Sechach.

The decorative sheet under the Sechach does not reduce the height of the Sechach (to reduce the 20 Amos in Kosher range or to reduce the 10 Tefachim into Pasul range).

(R. Ashi): But such vertical hangings do reduce the width of the Sukah.

R. Ashi's servant, Manimin, laid his wet coat to dry over the Sechach, and R. Ashi told him to remove it lest people think that he is using Sechach Pasul (Mekabel Tum'ah).

The servant asked if that is true even though the coat is obviously wet and is there to be dried, not as Sechach.

R. Ashi explained that he meant that the coat must be removed once it is dry.

There is a Machlokes whether a decorative sheet hung four Tefachim below the Sechach leaves the Sukah Kesheirah (R. Nachman) or renders it Pesulah (R. Chisda and Rabah bar R. Huna) [they differ on when the decoration is Batel to the Sechach].

In the reported incident, R. Nachman seated R. Chisda and Rabah bar R. Huna beneath the hung sheet in the home of the exilarch.

When they complied to sit there, R. Nachman asked them if they had retracted their earlier opposition to such a cover.

They responded that they were exempt from Sukah owing to their mission, and that no such inference can be drawn.

(R. Yehudah citing Shmuel): It is permitted to sleep under a covered Kilah (four posts, surrounding but unattached to the bed) in the Sukah, provided it is less than 10 Tefachim tall.

Question: The Beraisa teaches that one who sleeps in a Kilah has not fulfilled his obligation!?

Answer: That speaks of a Kilah taller than 10 Tefachim.

Question: But the Mishnah (2nd Perek) forbids sleeping under a bed!?

Answer: Shmuel already taught that the Mishnah there refers to a bed which sits above 10 Tefachim.

Question: But our Mishnah prohibits a Kinuf!?

Answer: That, too, is over 10 Tefachim.

Question: But the Beraisa does not seem to imply that it is speaking of Kinuf as being over 10 Tefachim!?

The Beraisa explains why, in our Mishnah, the Naklitin are permitted while the Kinufos are not permitted.

The Beraisa qualifies the permission regarding Naklitin as only if the top of the cover is less the 10 Tefachim taller than the bed, implying that the Kinufos are Pasul regardless of their height (which should apply to the Kilah the same way)!

Answer: Kinufos are different, and more stringent, because the posts are fixed on the bed, rendering it an Ohel.

Question: But a Sukah above a Sukah (which surely is well fixed on top) is still subject (according to Shmuel) to the rule of 10 Tefachim (and why should Kinufos be stricter)!?

Answer: To invalidate a Sukah requires 10 Tefachim of height (in the upper Sukah), but to create an Ohel (as in the Kinuf) takes less (so long as it is fixed).

(Shmuel): One who is sleeping in a Kilah unclothed may stick his head outside the Kilah and recite Shema (the Kilah is viewed as his clothes, not as an Ohel).

Question: But the Beraisa prohibits this practice!?

Answer: That is when the Kilah is taller than 10 Tefachim (and is thus considered an Ohel, and his head is viewed as being where his body is).

This is supported by the continuation of the Beraisa, which compares this person in a Kilah to one who is in a house.