Short answers, pulled from the story.
The Köchel catalogue is a comprehensive listing of all compositions by Wolfgang Amadeus Mozart, originally compiled by Ludwig Ritter von Köchel and first published in 1862. Each K. (or KV.) number identifies a specific work; for example, K. 626 designates the Requiem in D minor, which Köchel counted as Mozart's 626th composition.
Ludwig Ritter von Köchel created the catalogue, publishing the original 551-page edition in 1862. Earlier attempts at cataloguing Mozart's works, including those by Franz Gleißner and Johann Anton André (published in 1833), were less comprehensive.
There have been nine editions. The original appeared in 1862, with notable revisions in 1905-1937 (edited by Alfred Einstein), and 1964. The ninth edition, edited by Neal Zaslaw, was published in 2024 and abandoned chronological ordering, assigning works their oldest previous number to reduce confusion from decades of renumbering.
Einstein's third edition reassigned works from the original appendices into the main catalogue by inserting numbers with lower-case letter suffixes, drawing on analyses by Théodore de Wyzewa and Georges de St. Foix. This created fractional slots like K. 196e between existing numbers, a system that later caused widespread confusion when subsequent editions moved works again.
The earliest entries are small piano pieces Mozart composed at age five in Salzburg in 1761, including an Andante in C and a Minuet in F dated the 16th of December 1761. Mozart's own thematic catalogue of his compositions, however, only began in February 1784 with K. 449.
Three major complete-works sets have been released. Philips Classics Records issued a 180-CD set in 45 box sets between 1990 and 1991. Brilliant Classics released a 170-CD box in 2006 for Mozart's 250th birthday. Decca Classics and Deutsche Grammophon, in partnership with the International Mozarteum Foundation, released Mozart 225: The New Complete Edition on 200 CDs in 2016, including a new Mozart biography by Cliff Eisen and a short Köchel guide.