110. If the Seated Person was There First

18:19 If a person is sitting near someone who is saying Shemoneh Esrei because he is physically weak, he should not be rebuked.

18:20 If a person was already seated and someone else came along and started saying Shemoneh Esrei to the side of or in front of him, the seated person need not stand or move because the one praying entered his space (although it is meritorious to stand in such a case – Rema 102:3). However, if the person is praying behind him – that is, the seated person is in front of the one praying – then the seated person should stand up. All this is the case when praying at home but if in a shul or in another place of communal prayer, then the seated person must always stand since he was sitting in a place designated for prayer. (Mishnah Brurah 102:13 permits one who is studying Torah to remain seated.)