Gemara

(Beraisa): Not only the mourner's bed is inverted. Rather, all his beds (i.e. that he owns, even if he does not sleep on them) in the house are inverted!

(Beraisa): One need not invert a bed special for Kelim.

(Rav Yakov bar Acha): If a bed has poles jutting out at the head and foot, it suffices to stand it up.

Ta'anis 30b (Mishnah - R. Yehudah): One must invert the bed (on which he eats the last meal before Tish'ah b'Av);

Chachamim disagree.

(Beraisa - Chachamim): Pregnant and nursing women cannot bear this!

R. Yehudah: I obligate only one who is able.

(Beraisa): R. Yehudah agrees that one who cannot eat on an inverted bed is exempt. Chachamim agree that one who can eat on an inverted bed is obligated.

They argue about other beds in the house:

(Beraisa): Not only his (the Avel's) bed is turned. Rather, all his beds in the house are turned.

Mo'ed Katan 15a: An Avel must invert the bed;

(Bar Kapara - Beraisa): 'I (Hash-m) gave to them a semblance of My 'image'. Their sins caused Me to reverse it (from life to death). Therefore, their beds will be reversed.'

27a (Beraisa): If an Avel slept on a bed, chair, Odaini (a mortar, or a cover of a pit) or the ground, he was not Yotzei.

(R. Yochanan): He was not Yotzei inverting the bed.

Rishonim

Rif and Rosh (Mo'ed Katan 7b and 3:4): An Avel must invert the bed. 'I gave to you a semblance of My image... therefore, your beds will be reversed.''

Rif and Rosh (17a and 3:78): He inverts not only his bed, rather, all his beds in his house, even if he has 10 beds in 10 houses in 10 places.

Rosh: The Ra'avad says that this includes beds of his wife and children who mourn with him, but not beds for guests. Even though he owns them, since he does not sleep on them, they are exempt.

Question (Ramban, brought in Rosh): We need not teach about beds of his wife and children who mourn (with him), for they themselves are obligated. Rather, it teaches about beds of guests. If guests come, he erects them, and inverts them when they leave. Also, we exempt a bed for Kelim, which is like a mere box. If a bed on which people sleep was exempt, we should have taught it!

Answer (Rosh): We teach that beds of his wife and children, who merely mourn for his honor, must be inverted. It is a Chidush that a bed for Kelim, which serves him, is exempt, and all the more so beds that serve guests. Presumably, the Ra'avad is correct.

Nimukei Yosef (DH Ela): The Ra'avad exempts from inverting beds for guests. We learned that if the consolers are not so informal with the Avel, they sit on erect beds when giving to him his first meal (26b).

Rosh (ibid.): The obligation to invert 10 beds in all 10 houses is when he sometimes sleeps on them, even if he sleeps primarily in the house of the Mes. Avel Rabsi (1) teaches that he inverts beds only in houses that he uses. One who is lodging with the Avel inverts his bed only if he is informal with the Avel. The Yerushalmi exempts a guest in an inn from inverting the bed, lest they say that it is witchcraft. People in Ashkenaz and France rely on this not to invert the beds, for they live among Nochrim, and the Nochrim are frequent in their houses. The Ritzva says that our beds stick out at the ends, unlike those in the days of the Gemara, so it would not be recognized if our beds were inverted. Therefore, inverting does not apply to them.

Rif and Rosh (ibid.): If one of five brothers died, all of them invert all of their beds. One need not invert a bed special for Kelim. If a bed has poles jutting out at the head or foot, it suffices to stand it up. If an Avel slept on a bed, chair, Odaini, or the ground, he was not Yotzei inverting the bed.

Rosh: If he inverted his bed and slept on the ground, he was Yotzei. The Mitzvah is to invert the bed, not to sleep on an inverted bed.

Nimukei Yosef (DH Gedolah): This is like Yerushalmi. The Ra'avad agrees. However, the Rambam requires him to sleep on an inverted bed.

Rambam (Hilchos Evel 5:18): An Avel must invert the bed all seven days. He must invert all his beds in the house, even if he has 10 beds in 10 houses in 10 cities. If one of five brothers died, all of them invert all of their beds. One need not invert a bed special for Kelim or coins. If a bed has poles jutting out at the head and foot, it is impossible to invert it. It suffices to stand it upright.

Nimukei Yosef (Mo'ed Katan 17a DH Naklitin): Other beds can be totally inverted, so one is not Yotzei by tilting them on the side.

Rambam (ibid.): If he inverted all his beds and slept on others' beds, a chair, chest or the ground, he was not Yotzei. He must sleep on an inverted bed.

Rashi (Ta'anis 30b DH She'ar): Chachamim and R. Yehudah argue about Erev Tish'ah b'Av, and about whether or not an Avel must invert all beds in the house.

Poskim

Shulchan Aruch (YD 387:1): An Avel must invert the bed.

Tur: The Chiyuv applies to a wooden bed woven with ropes. He must put the top down and the legs up. Even if it is on benches or rocks and has three or four sheets on it, as long as the legs face up he was Yotzei. This is not in order to pain him, in which case we would say that if he slept on the ground, or on the ropes without a sheet, he was Yotzei. Rather, it is because 'I gave you a semblance of My image... therefore all beds will be reversed.' Therefore, if he reversed it, even if he slept on the ground and not on the bed, he was Yotzei. The Rosh holds like this. The Rambam says that he was Yotzei only if he slept on an inverted bed.

Beis Yosef (DH veha'Ra'avad): Since the Ra'avad, Ramban and Rosh agree, we follow them against the Rambam, especially since the Halachah follows the lenient opinion in Aveilus.

Shulchan Aruch (ibid.): When he sleeps and eats, he sits on an inverted bed. The entire day he does not sit even on an inverted bed, rather, on the ground. The consolers may sit only on the ground.

Rebuttal (Gra 1): Toras ha'Adam proves from the Yerushalmi that (regarding Aveilus of an individual,) if he inverted the bed, he may sleep on a bench, and all the more so on the ground. The Shulchan Aruch is difficult.

Shulchan Aruch (2): Nowadays we do not invert the bed, lest Nochrim say that it is witchcraft. Also, our beds are unlike those in the days of the Gemara, and it would not be recognized that they are inverted.

Bedek ha'Bayis (DH Kosav): The Rivash (69) says that our beds cannot be inverted. The Gemara said that if poles jut out and make it impossible to invert the bed, it suffices to stand it up, i.e. on its side. Similarly, our beds cannot be inverted.

Shach (1): Even though we do not invert the beds, the Avel may not sit on a bench, or on pillows and blankets, only on the ground. If he is old or sick and it is painful to sit on the ground, he may have a small cushion underneath.