Requiem (Mozart) in the context of Köchel catalog

The Köchel catalogue (German: Köchel-Verzeichnis) is a catalogue of compositions by Wolfgang Amadeus Mozart, originally created by Ludwig Ritter von Köchel, in which the entries are abbreviated K. or KV. Its numbers reflect the ongoing task of compiling the chronology of Mozart's works, and provide a shorthand reference to the compositions. For example, according to Köchel's counting, Requiem in D minor is the 626th piece Mozart composed, thus is designated K. 626.

Köchel's original catalogue (1862) has been revised several times. Catalogue numbers from these revised editions are indicated either by parentheses or by superscript: K. 49 (47d) or K. 47d refers to the work numbered 47d in the sixth edition. The catalog was originally chronological, though revisions made chronological ordering of Mozart's works difficult and as of 2024 a new organizational system is used for the ninth version.

Music for the Requiem Mass is any music that accompanies the Requiem, or Mass for the Dead, in the Catholic Church. This church service has inspired hundreds of compositions, including settings by Victoria, Mozart, Berlioz, Verdi, Fauré, Dvořák, Duruflé and Britten. For centuries settings of the Mass for the Dead were to be chanted in liturgical service monophonically. Later the settings became polyphonic, Victoria's famous 1605 a cappella work being an example. By Mozart's time (1791) it was standard to embed the dramatic and long Dies irae (Day of Wrath) sequence, and to score with orchestra. Eventually many settings of the Requiem, not least Verdi's (1874), were essentially concert pieces unsuitable for church service.