A Sukah Meduvleles (to be explained) and/or whose shade is greater than its sun is/are Kesheirah.

If the Sechach is very thick (even though stars cannot be seen from within) the Sukah is Kesheirah.

Question: What is Meduvleles?

Answer (Rav): A 'poor' Sukah (thin Sechach).

Answer (Shmuel): One whose Sechach is 'mixed,' laid one piece high and one low.

Rav taught 1.a. (above) as one case (it is Kosher even though it is thin provided that its shade exceeds its light).

Shmuel taught it as two cases (mixed as one Kosher case; shade exceeding light as a second Kosher case).

(Abaye): The Heter of Shmuel only applies if the higher and lower pieces are not separated by three Tefachim.

(Rava): The Heter applies even if they are three Tefachim apart, provided that each plank is a Tefach wide.

If less, then they need to be within three Tefachim of one another.

This Heter is based on the principle of Chavot Rami.

(Rava): My source for limiting Chavot Rami to a board whose width is one Tefach is the Mishnah in Ohalos and the Beraisa taught regarding that Mishnah.

The Mishnah provides the Halachah in every permutation where a two-story structure has beams in the ceiling of each story.

Tum'ah is either allowed to flow upward or is blocked by the Ohel created by the boards.

If the boards of the two stories are aligned with one another, then Tum'ah rises unimpeded; if the upper boards are above the spaces between the lower boards, then Tum'ah anywhere in the building is trapped as though it were all one room.

The Beraisa qualifies the Mishnah as applying when the boards and the spaces between them are a Tefach wide, in order to allow for Chavot Rami.

Question (R. Ashi of R. Kehana who had just reported Rava's assertion): But we see in the Beraisa that there is Chavot Rami even with less than a Tefach width!?

The Beraisa permits incomplete Koros at the entrance to a Mavoy with three (or, according to R. Shimon b. Gamliel, four) Tefachim of space in between.

Twin Koros, each less than the minimum Tefach width, may be combined.

R. Shimon b. Gamliel permits a space between the two twin Koros, provided they are strong enough to hold an Ariach (half-brick).

If the two twin Koros were at a different height, then R. Yosi b'Rebbi Yehudah teaches that we view them as at the same height, provided that the upper Korah is not above 20 Amos, and the lower one is not below 10 Tefachim.

It would thus appear that we are permitting Koros of less than a Tefach to combine through Chavot Rami!

Answer (R. Kehana): The Koros must be within three Tefachim of one another (using Lavud, since we do not say Chavot Rami with such narrow boards), and the Beraisa is only reminding us of the minimum and maximum permitted height for Koros.

Question: Our Mishnah implies that were the shade and light to be equal, the Sukah is Pesulah, while the earlier Mishnah implies that equal shade and light is Kesheirah!?

Answer: It is Pesulah if equal at the Sechach level and Kesheirah if it is equal at the ground-shade level (since the shade must be greater at the Sechach level).

A Sukah is still Kesheirah even if the stars are not visible.

If sunlight does not penetrate the Sechach it is Pasul according to Beis Shamai and Kosher according to Beis Hillel.

A Sukah built on a wagon or boat is Kesheirah.

On a tree or a camel is Kesheirah, but may not be entered on Yom Tov.

If one or more of its walls are in the tree, even if the second or third are on the ground, it is viewed as in the tree and may not be entered on Yom Tov.

If three walls are on the ground we are not concerned if the fourth wall is supported by the tree.

The Sukah is tested by checking if it could stand without the tree.