It is permissible to change an obligatory prayer into an unrestricted voluntary prayer, whereas the reverse is not valid. (1)

There is consensus that one does not commit disbelief by abandoning a voluntary prayer. As for the obligatory prayer, however, abandoning it constitutes disbelief according to the sounder view. (2)

Voluntary prayers make up for deficiencies in obligatory prayers, whereas the reverse is not valid.

Standing is an essential pillar of an obligatory prayer, unlike a voluntary prayer.

A runaway slave’s voluntary prayer is invalid, whereas his obligatory prayer is valid.

According to one of the two views, it is permissible to suffice with a single salutation of peace in a voluntary prayer, but not in an obligatory prayer. (3)

The call to prayer and the iqamah are not prescribed at all for voluntary prayers, unlike obligatory prayers.

An obligatory prayer may be shortened while travelling, but a voluntary prayer offered while travelling is not shortened.

A voluntary prayer is waived when one is unable to perform it, and its reward is recorded for one who was accustomed to observing it. An obligatory prayer, however, is never waived; and the reward for completing it is recorded for one unable to do so, if he was accustomed to performing it.

Remembrance after the prayer is prescribed for all obligatory prayers. As for voluntary prayers, it has been reported for some of them, but not for others.

A voluntary prayer may be performed inside the Ka‘bah, whereas an obligatory prayer may not. The sound view, however, is that it is permissible; thus, there is no difference between them. (4)

Congregational prayer is obligatory for obligatory prayers, but not for voluntary prayers.