The Köchel catalogue (German: Köchel-Verzeichnis) is a chronological 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.

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; catalogue numbers from the sixth edition are indicated either by parentheses or by superscript: K. 49 (47d) or K.⁶ 47d.

In the decades after Mozart's death there were several attempts to catalogue his compositions, for example by Franz Gleißner and Johann Anton André (published in 1833), but it was not until 1862 that Ludwig von Köchel succeeded in producing a comprehensive listing. Köchel's 551-page catalogue was titled Chronologisch-thematisches Verzeichniss sämmtlicher Tonwerke W. A. Mozart's (Chronological-thematic Catalogue of the Complete Musical Works of W. A. Mozart).^[1] Köchel attempted to arrange the works in chronological order, but many compositions written before 1784 could only be estimated, although Leopold Mozart had compiled a partial list of his son's earlier works; Mozart's catalogue of his own compositions (begun in February 1784 with K. 449) allows relatively precise dating of many of his later works. The catalogue included the opening bars of each piece, known as an incipit. Köchel divided the corpus into a main chronology of 626 works, and five appendices (Anhänge in German), abbreviated Anh. I–V which comprise:

• I – Lost authentic works
• II – Fragments by Mozart
• III – Works by Mozart transcribed by others
• IV – Doubtful works
• V – Misattributed works

Since Köchel published his original catalogue in 1863 (now referred to as K¹), the dating of Mozart's compositions has been subject to constant revision. Many more pieces have since been found, re-dated, re-attributed and re-numbered, requiring three revised editions of the catalogue. Subsequent editions – especially the third edition (K³) by Alfred Einstein (1937), and the sixth edition (K⁶) by Franz Giegling, Gerd Sievers [de], and Alexander Weinmann [de] (1964) – have reflected attempts to arrange the growing list of works in a more accurate chronological order, according to various levels of scholarship.

A major shortcoming of K¹ was that there was no room to expand the strictly sequential numbering in the main catalogue to allow for any new discoveries or further reassessment of existing works. For the 1937 edition (K³) Einstein (following the analyses of Théodore de Wyzewa and Georges de St. Foix) reassigned some works from the original K¹ appendices into the main catalogue by interpolating numbers with a lower-case letter suffix. In K⁶ some of these were, per intervening scholarship, returned to re-structured appendices:

• K. 626a
  • K. 626a I – 64 cadenzas by Mozart to his own keyboard concertos
  • K. 626a II – Cadenzas by Mozart to keyboard concertos by other composers
• K. 626b – 42 sketches & other fragments by Mozart (replacing K³ Anh. II)
• Anh. A – Copies by Mozart of other composers' works
• Anh. B – Works by Mozart transcribed by others
• Anh. C – Doubtful and misattributed vocal (C 1–10) and instrumental (C 11-30) works

For example, Divertimento for Wind Octet in E♭ was numbered Anh. 226 in K¹; Einstein placed it in the K³ main catalogue as K. 196e, between K. 196 and K. 197; K⁶ reassigned it again to the 'doubtful' appendix C as Anh. C 17.01. Some works in Anh. A have been identified since 1965 as by Leopold Mozart. Many works in Anh. C have since been more reliably assigned to other composers, or to Mozart himself.

To maintain as much of the original K-numbering of the list as possible, while re-ordering in the revised, chronological sequence, letters were added to the new numbers. The following list shows Köchel's original designation (K¹) and the sixth edition (K⁶). Other addenda and supplements to the catalogue are marked Anhang. (This has been abbreviated to 'Anh.' in the list below.)