This Halacha is an excerpt from our Sefer
Sitting on the ground:
On the night of Tisha B’av, and by day, one sits on the floor in Shul [and in all other places] until the time of Mincha. However, today the custom is to sit on benches immediately after leaving Shul after Shacharis, and the reading of Kinos, which is to be lengthened until sometime near midday. [Practically, one is not to sit on a bench until midday.]
May one sit on a cushion? One is not required to sit directly on the floor, and he may thus sit on a cushioned bottom. [Furthermore, based on Kabala, one is to have an interval between him and the floor as explained in the Q&A.]
May one sit on a low stool? It is permitted to sit on a low stool if it is difficult for one to sit on the ground. [Today the custom is for all people to be lenient in this matter. Some are particular that the stool be within three Tefach from the ground-see Q&A!]
Must one sit on the ground or may he choose to stand? It is not required for one to sit on the ground and he may stand if he so chooses.
The custom is to sit on the floor and not on a chair or bench from the beginning of Tisha B’av until midday.
May a pregnant woman or woman after birth, sit on a chair if it is difficult for her to sit on the floor?
May a nursing woman sit on a couch or bed if it is difficult for her to nurse on the floor?
May an old person sit on a chair if it is difficult for him to sit on the floor?
Yes, as explained above.
May one sit while on a bus or in a car?
How low must one’s stool be?
May one sit directly on the floor?
No. Based on Kabala one is never to sit directly on the ground and is rather to have an interval between him and the floor. Clothing that one is wearing is not considered a valid interval for this matter. One should be stringent even regarding a tiled floor.
May one sit on a pillow or cushion?
Yes, as explained above.
May one sit by the steps leading to the Aron?
 So is the custom in all places and not just in a Shul [See Piskeiy Teshuvos 559 footnote 14; Beis Yosef 559] and so is the ruling of the following Poskim who do not record the word Shul in their ruling: Chayeh Adam 135:20; Derech Hachaim T”B 8; Kitzur SHU”A 124:16
 Michaber ibid; Hagahos Maimanis “The custom of the French is not to sit on benches until Mincha”
The reason: This is similar to a mourner which sits on the floor throughout his seven days of mourning. [Beis Yosef in name of Hagahos Maimanis; Taz 559:3; M”B 559:10] The reason this custom is only kept until the time of Mincha is because it is merely a custom and is not required from the letter of the law, in contrast to the other mourning customs that must be observed until nightfall. [Bach 559; Taz 559:3; M”B 559:10]
 Rama ibid
 Taz 559:3; M”A 559:3 “As they go to the cemetery after Kinos and hence by the time they return it is already midday and hence permitted to sit on a bench”; M”B 559:12
Other opinions: The above follows the ruling of the Rama, however according to the Michaber ibid one must sit on the floor until the time of Mincha. Some Poskim rule that one may be lenient to sit on a bench [and not chair] from after the completion of Kinos even if it is prior to midday. [Nitei Gavriel 68:7 in name of Hagahos Baruch Taam 559]
 M”A 559:2; so rules also: Machatzis Hashekel ibid; P”M 559 A”A 2; Chayeh Adam 135:20; M”B 559:11; Kaf Hachaim 559:22
Other Opinions: The Maharil was accustomed to sit directly on the ground. He would thus wear raggedy clothing until the time of Mincha in order so his good clothing do not become damaged. [Drashos Mahril Hilchos Tisha B’av; Kneses Hagedola 559:12; Elya Raba 559:1; Beir Heiytiv 559:3; Kaf Hachaim 559:21] Some learn that this was a mere stringency. [Kaf Hachaim 559:22] Others however learn that the Maharil held it was forbidden due to the custom. [See P”M ibid for how he explains the Maharil]
 The reason: As this matter of sitting on the floor is a mere custom. [M”A ibid]
 Mishneh Berurah 559:11; Kaf Hachaim 559:22
 Piskeiy Teshuvos 559:4 in name of Yalkut Hagershoni; Mishnas Yaakov 559; as is also the custom today by an Avel today. [Gesher Hachaim 20:5-11]
 Taz 559:3
 Aruch Hashulchan Yoreh Deah 387:3 regarding Aveilus of a pregnant woman; Toras Hayoledes 48:12; Piskeiy Teshuvos 559:4;
 As they are not doing so for pleasure but simply to remove discomfort.
 Nitei Gavriel 68:11
 As stated above
 Piskeiy Teshuvos 559:4 in name of Toras Hayoledes 48 footnote 17; Rav Elyashiv; Rivivos Efraim 1:382
 As one is doing so for safety reasons and not for comfort.
 Kuntrus Iyun Tefila; Poskim brought in Nitei Gavriel 68:3; Piskeiy Teshuvos 559:4
 Custom of Rebbe brought in Hiskashrus; Piskeiy Teshuvos ibid footnote 18 in name of Steipler; and so is the implication of the M”B who does not write how low it is to be.
 Maharash Shaar Aryeh; Birkeiy Yosef 552:8; Shaareiy Teshuvah 552:3 in name of Birkeiy Yosef ibid; Kaf Hachaim 552:39
Other opinions: From the Maharil brought in Beir Heiytiv 559:3 it is implied he held that one may sit directly on the ground. [Shaareiy Teshuvah ibid] Practically however one is to be stringent in this matter. [Shaareiy Teshuvah ibid; see P”M 559 A”A for how he explains the Maharil; See also next footnotes from Kaf Hachaim which differentiates between a tiled and earth floor]
 Shaareiy Teshuvah 552:3 in name of Birkeiy Yosef
 Kaf Hachaim ibid suggests that perhaps the above stringency of Kabala is only by an earth floor and thus there is no contradiction between the Maharil and the Maharash however he concludes “When possible one is to be stringent even regarding a tiled floor.”
 Piskeiy Teshuvos 559:4 in name of Makor Chaim