Pope Pius XI's body was carried to St. Peter's Basilica for exposition and a Requiem Mass.

A Requiem is a Mass celebrated for the repose of the soul(s) of a deceased person(s), using a particular form of the Roman Missal..

The term is also used for similar ceremonies outside the Roman Catholic Church, both  the Anglo-Catholic branch of Anglicanism and the Lutheran Church.  A comparable service with a different ritual form and texts, exists in the Eastern Orthodox and Eastern Catholic Churches. There is even a Requiem for the Methodist Church by John Rutter.

The Mass and its settings draw their name from the introit of the liturgy, which begins with the words "Requiem aeternam dona eis, Domine" – "Grant them eternal rest, O Lord". ("Requiem" is the  accusative singular form of the Latin noun requies, "rest, repose".) The Roman Missal as revised in 1970 employs this phrase as the first entrance antiphon among the formulas for Masses for the dead, and it remains in use.

The Requiem Mass is notable for the large number of musical compositions that it has inspired.  Originally, such compositions were meant to be performed in liturgical service, with monophonic chant. Eventually the dramatic character of the text began to appeal to composers to an extent that they made the requiem a genre of its own, and the compositions of composers such as Verdi are essentially concert pieces rather than liturgical works.

Many of the texts in the Requiem Mass have been set to music, including: