Requiesce in pace, the latin for rest in peace. The phrase in English was not found on tombstones before the eighth century. It became common on the tombs of Catholics in the 18th century, for whom it was a prayerful request that their soul should find peace in the afterlife. When the phrase became conventional, the absence of a reference to the soul led people to suppose that it was the physical body that was enjoined to lie peacefully in the grave. This is associated with the Catholic doctrine of the particular judgment; that is, that the soul is parted from the body upon death, but that the soul and body will be reunited on Judgment Day.
Other variations include "Requiescat in pace et in amore" for "May she rest in peace and love", and "In pace requiescat et in amore". The word order is variable because Latin syntactical relationships are indicated by the inflexional endings, not by word order. However, if "Rest in peace" is used in an imperative mood, it would be "Requiesce in pace" (acronym R.I.P.) in the second person singular.