According to [the letter of] the law, there is no obligation to sleep or lie on the ground on Tish’a B’Av. For Chazal’s statement – “All mitzvot that apply to a mourner apply on Tish’a B’Av” (Ta’anit 30a) – refers only to those acts that are forbidden during the [seven-day] mourning period, like washing, anointing, wearing shoes, marital relations, greeting one another, and Torah study. However, the mitzvot that a mourner must keep, like turning over the beds and sitting on the floor, do not apply on Tish’a B’Av according to [the letter of] the law (Tur, O.C. 555). Nonetheless, the custom is to exhibit our mourning on Tish’a B’Av in the way we lie and sit, as well. But since this law is based on custom, it is more lenient [than the other prohibitions are], as we will explain presently.
Lying down: Some people sleep on the ground; others sleep without a pillow; and still others place a stone beneath their heads (Sh.A. 555:2). One who finds it difficult to sleep in this manner, may sleep normally (M.B. 555:6). The prevalent custom is to… place one’s mattress on the ground, thus precluding the need to remove one’s pillow. It is best to place a stone underneath the mattress. This way, one observes the custom of mourning without having much difficulty falling asleep.
Sitting: The custom is to sit on the ground like mourners. However, since there is no halachic obligation to do so, we are not strict about this all day long (Bach 559:1). Ashkenazim [sit on the ground] until midday, while Sefardim [do so] until the Minchah (Afternoon) service (Sh.A. and Rama 559:3). Thus, one who takes a nap in the afternoon need not place his mattress on the ground.
We already learned (above, 9.3) that some have a custom, based on Kabbalah, not to sit on the ground without a piece of cloth or wood to separate [between the person and the floor] (Birkei Yosef 555:8). If the floor is tiled, however, many [poskim] maintain that there is no need [for a separation], even according to Kabbalah. Some are meticulous to make a separation even on tiled floors, but many people follow the lenient custom.
Since there is no halachic obligation to sit on the ground, one may sit on a small cushion or a low bench, but they should preferably be no higher than a tefach (handbreadth) off the ground. If it is difficult to sit so low, one may be lenient and sit on a chair that is less than three tefachim (24 cm) high. And if even this is difficult, one may sit on a chair that is slightly higher than three tefachim.
Sitting on stairs is considered sitting on the ground, because people step on them (Mekor Chayim by the [author of] Chavot Ya’ir). Some [authorities] allow one to sit on an overturned stender (lectern), even if it is higher than three tefachim. Since it is not designated for sitting, one who sits on it is not considered as one who sits on a chair. Pregnant women, the elderly, the sick, and those who suffer from back aches – for whom sitting on a low chair is difficult – may sit on regular chairs (A.H.Sh., Y.D. 387:3).