Performing salah is obligatory for all adult Muslims, with a few dispensations for those for whom it would be difficult. To perform valid salah, Muslims must be in a state of ritual purity, which is mainly achieved by ritual ablution according to prescribed procedures. The place of prayer should be clean. In a few cases where blood is leaving the body, salah is forbidden until a later time.
Salah consists of the repetition of two or more units of a prescribed sequence of actions and words. One complete sequence is known as a rak'ah (pl. rak'āt). The number of obligatory (fard) rak'at varies according to the time of day or other circumstances (such as Friday congregational prayers). Additions to the fard rak'at can be made, again in different multiples according to the circumstances. These are not required, but are considered meritorious. There are also dispensations from some or all of the prescribed actions for those who are physically unable to complete them. The prescribed words of the prayer remain obligatory.
Salah is prescribed at five periods of the day as part of tradition, which are measured according to the movement of the sun. These are: near dawn (fajr), just after the sun's noon (dhuhr), in the afternoon (asr), just after sunset (maghrib) and around nightfall (isha'a). Under some circumstances prayers can be shortened or combined (according to prescribed procedures). Prayers can be missed in serious cases, but they should be made up later.