Gemara

R. Meir needs a tradition for Gud [Achis or Gud Asik, i.e. to consider a wall to extend downward or upward], Lavud (if a gap is less than three Tefachim, it is considered to be filled in), and Dofen Akumah.

10b (Beraisa): Hide of a toilet seat and its hole join to the Shi'ur of a Tefach [to be an Ohel].

(Rav Dimi): The hole can be [up to] two fingers, with two fingers [of hide] on each side.

(Ravin): It can be [up to] one finger, with one and a half fingers on each side.

Rav Dimi: We argue about Parutz k'Omed (there is as much air as solid wall. I permit, and he forbids.)

Shabbos 8a (Abaye): If one threw a basket 10 Tefachim tall [from Reshus ha'Yachid] to Reshus ha'Rabim, he is liable only if its width is less than six Tefachim wide.

(Rava): He is exempt even if it is less than six.

Surely, strands extend above 10 Tefachim, so part of the basket is in Mekom Petur.

(Rava): If he turned it over [so the opening faces down] if it is seven and a half tall, he is exempt.

(Rav Ashi): Even if it is seven and a half, he is liable.

This is because walls of a Kli are made for the inside.

101a (Beraisa - R. Yosi b'Rebbi Yehudah): If one inserted a pole in Reshus ha'Rabim with a Traskal (a basket four by four wide) on top, and one threw something (from Reshus ha'Rabim) and it landed on the basket, he is liable.

The basket is Reshus ha'Yachid due to Gud Achis.

(Rav Yosef): Rav Yehudah taught that Chachamim [argue with R. Yosi b'Rebbi Yehudah, and] exempt!

Question (Abaye - Beraisa): If a pillar in Reshus ha'Rabim is 10 tall and four wide, but at the bottom it is less than four wide, [even] if the narrow part is three Tefachim tall [so Lavud does not apply], if one threw [from Reshus ha'Rabim] and it landed on top of it, he is liable.

Inference: We say Gud Achis!

Regarding the basket Chachamim do not say Gud Achis because goats can stick their heads through. This does not apply to the pole. (Goats would not come close to scratch themselves on it.)

Rishonim

Tosfos (8a DH Shiv'ah): If the basket is seven and a half tall, when the walls are within three Tefachim of the ground, there is a Mechitzah 10 Tefachim tall even without the bottom of the Kli. It is considered a Reshus ha'Yachid, and he is exempt [because part is Munach (resting) in Mekom Petur]. He is exempt even later when it falls to the ground and ceases to be Reshus ha'Yachid. However, if it is less than six wide, he is liable. Lavud applies only to Mechitzos, and the Mechitzos are important only if they enclose [a square] four [by four Tefachim, so the basket must be about six wide]. Therefore, it is like other objects, and Lavud does not apply.

Ramban (8a DH Kaf'ah): Some Tosafists are Mechayev when it is not six wide, because Lavud applies only to important Mechitzos that enclose four. It is like other objects, and Lavud does not apply. I say that Lavud never applies to Kelim. Rather, he is exempt because when it is within three, it is considered Munach, and it is Agud (partially in a Reshus) above 10. It is not due to Mechitzos. The Rif did not specify the width, for it does not depend on this, like I wrote in Milchamos Hash-m (below).

Ba'al ha'Ma'or (Shabbos 2b): The Rif did not explain throwing an inverted basket. Rashi explained it well. If it is six wide, if it is Mashehu more than seven tall, he is exempt, for it is a Reshus by itself. Once it is within three of the ground, it is as if it is Munach, for we say Gud Achis (we extend the wall down), and it is as if it reached the ground. This is only when he inverted it, and the [open] edge of the Mechitzos faces down. If it faces up, since the bottom faces down and the [open] edge faces up, and not down, we do not say Gud Achis, and it is not as if it reached the ground.

Milchamos Hash-m: The Ba'al ha'Ma'or says that it must be less than six wide. Tosfos says that it must be more than six. Really, in every case, Lavud does not make a Kli a Reshus. We do not find Lavud, Gud Asik and Gud Achis regarding Kelim. Rather, he is exempt because when it is within three, it is considered Munach. One opinion holds that Rabanan require Hanachah on something. That refers to throwing from one place to another place where it will not finally rest. Here, this is like Moshit (passing), for it will rest here. The Hanachah began once it was within three. One is liable if it is less than seven and a half tall, since it does not become a Reshus [and Hanachah is in Reshus ha'Rabim - PF]. Rav Ashi says that the walls are for the sake of the inside, so we do not lower them [to consider the Kli Munach]. Rather, we consider it Munach within three, as if Reshus ha'Rabim ascends. This explains [liability for] a normal case of throwing. This is why Rava did not specify how wide it is [when he threw it inverted].

Question (Rashba 8a DH veha'Ramban): If so, why did the Gemara need to say that he inverted it? The same applies if he threw it normally! Perhaps if he did not invert it, he is exempt even if it is seven and Mashehu, for strands will extend above 10. The bottom is smooth, and strands do not extend above. However, it is still difficult, for Rava must exempt due to Lavud, since Rav Ashi said "even if it is seven and a half, he is liable, because walls of a Kli are made for the inside." I.e. they are not Mechitzos to which Lavud applies. This shows that Rava exempts due to Lavud. Also, if Rava exempts due to Munach, why is Rav Ashi Mechayev? Do they argue about whether or not within three is Munach?!

Rashba (Eruvin 10b DH Iy): Lavud applies only to land or something stationary, but not to movable Kelim. I explain that this was Rav Ashi's conclusion, that Lavud does not apply to Kelim, because the Mechitzos are made only for use.

Ritva (10b DH Ha): Some say that Lavud does not apply to Kelim. Shabbos 8a connotes that Lavud applies to Kelim! I answer that Lavud does not apply to something made to be always open.

Pnei Yehoshua (Shabbos 9a DH b'Perush): The Rambam holds that Gud Achis and Lavud do not apply to Kelim. This is astounding. On 5a, it is clear that Gud Achis applies to Kelim! Tosfos says that Traskal (a basket) is a Kli, and we say that R. Yosi b'Rebbi Yehudah obligates [one who throws into it] due to Gud Achis! Rabanan argue only because goats can go through. Normally, they agree! If so, also Lavud applies to Kelim. We can similarly challenge Rav Ashi, who is Mechayev because the walls are for the Kli [so Lavud does not apply]. How can he argue with the Beraisa of a basket? Rav Ashi could answer that the Beraisa discusses a basket built onto a beam, so it ceases to be a Kli. However, Tosfos calls it a Kli, and derives that Karmelis does not apply to Kelim. This requires great investigation.

Rivash (83,125): The Rashba says that any amount of Hamshachah suffices. (One pours water onto the ground, and it flows into a deficient Mikveh. This helps, even though the water was in a Kli.) I require that the water flow at least three Tefachim on the ground, to avoid Lavud.

Poskim

Shulchan Aruch (YD 201:45): The Shi'ur of Hamshachah is three Tefachim.

Shulchan Aruch (OC 87:3): One may read Shma in a house with excrement if he covered it with a Kli. It is as if it is buried.

Magen Avraham (4): If a Kli for excrement (e.g. a potty) was under a bed, and the walls of the bed reach within three Tefachim of the ground, one may read Shma nearby. See what I wrote in 502:9 about whether Lavud applies to Kelim. Here all agree that it is considered sealed. Therefore, the same applies if he covered it with a Kli, and there is a little air between the Kli and the ground, e.g. it sticks out on one side. However, if the Kli is open above it is forbidden, for the excrement must be covered.

Levushei Serad (9): Here it suffices, for the excrement is partially covered. He wrote differently in 502. There, he proved from here that Lavud applies to Kelim! One can resolve this.

Machatzis ha'Shekel: With difficulty, we can say that also there he means that [even if Lavud does not apply, it suffices here, for] it is partially covered.

Bi'ur Halachah (DH Od): If a Mechitzah is 10 Tefachim tall and four wide, even if it is less than four wide within three of the ground, it is a good cover due to Lavud.

Rema (502:1): If a table has walls that reach the floor, it requires a Shinuy [to erect it, e.g. to hold the tabletop in place, and put the legs under it. One may not assemble it normally.]

Beis Yosef (DH u'Mah she'Chasav Al): If the walls reach within three Tefachim of the floor, it looks like they touch the floor, for anything within three is Lavud.

Magen Avraham (9): This depends on the argument of whether Lavud applies to Kelim. Rashi and Ba'al ha'Ma'or apply Lavud to Kelim less than four by four, unless the walls are made to put things inside. Tosfos Lavud only to Mechitzos that are seven tall without the bottom, and four wide. If not, it is like other matters, and Lavud does not apply. The Ramban says that Lavud and Gud do not apply to Kelim. Shabbos 101a and Eruvin 33a connote unlike this The Tur (87) applies Lavud to a bed. Therefore, I say that one should be stringent.

Machatzis ha'Shekel (DH Piresh Rashi, on 79b, top of second column): Rashi exempts when it is considered a Reshus through Lavud, for in the Mishkan they threw only Kelim, but not Reshuyos. Tosfos applies Lavud to Kelim only if through this they become a Reshus. The Ramban applies Lavud to Kelim, but this does not make the Kli a Reshus. Rashi holds that Lavud can make a Kli a Reshus. Similarly, it can make a Kli an Ohel, even if the walls of the table are Mashehu more than seven and the top is only a [square] Tefach. Tosfos holds that Lavud does not apply to Kelim [unless the Kli becomes a Reshus]. Therefore, one may erect the table unless the top is four by four [and the walls are more than seven]. The Ramban holds that a Kli never becomes a Reshus through Lavud, so in every case one may erect the table.

Machatzis ha'Shekel: The Magen Avraham challenged the Ramban, for 101a proves that a Kli can become Reshus ha'Yachid. I do not understand his proof from Eruvin, for there the Kli is right next to the wall. What is the relevance to Siman 87? There, the covering need not be considered a Reshus!

Tzemach Tzedek (OC 8 DH Emnam): Also Shabbos 101a is not difficult for the Ramban. It becomes Reshus ha'Yachid due to Gud Achis [and not due to Lavud].

Note: The Ramban said that neither Gud Achis nor Lavud applies to Kelim!

Chasam Sofer (YD 199 DH Ach): Tosfos proved that when the Mechitzah is four wide, Lavud applies.

Beis She'arim (OC 131): It seems that Rashi held like the Ramban, that Lavud and Gud do not apply to Kelim. He did not want to explain 8a like the Ramban. Rather, it is not a Kli, for it is a Reshus by itself.

Beis She'arim (OC 257): We say that something within three Tefachim of the ground is like Munach. This is not due to Lavud. Rather, it is like an extension of the ground, for the ground is not level everywhere.