However, in addition to botany, he also loved music, and this led to his personal discovery of Mozart’s music, and the lack of any clear-cut catalogue of the works of this master. To this time, such efforts had been incomplete at best, or worse, incorrect AND incomplete, sometimes painfully so. Becoming a member of the Mozarteum not long after its founding, he determined to rectify the obvious lack of a correct listing of the true, complete works of Mozart. Inspired by the on-going publication by the publishers Breitkopf & Härtel of Otto Jahn’s magnificent four-volume biography of Mozart (publishing ran from 1856-59), Köchel agreed with these publishers to research and produce a complete catalogue of all of Mozart’s works.

Mozart's work had gone through several cataloguers, beginning with Abbe Maximilian Stadler (who was assisted by Constanze's second husband, Georg Nikolaus von Nissen), furthered by Joseph Haydn and Johann Anton Andre’, as well as Breitkopf & Härtel themselves early on. To be truthful, it may not have been possible before this time to produce such an accurate work. Many manuscripts and facsimiles had been temporarily lost or neglected, and previous chroniclers simply did not have the resources to pursue such a daunting task, the breadth of which could only be guessed at. Certainly no previous attempt had the talent and abilities Köchel brought to the task. Finally the magisterial work was published in1862 as: Chronologisch-thematisches Verzeichnis sämtlicher Tonwerke Wolfgang Amadé Mozarts, a chronological and thematic register of the works of Mozart. It is known informally today as "the Köchel catalogue".

This catalogue was the first on such a scale and with such a level of scholarship behind it. Köchel arranged this opus such that the works were placed in chronological order, but the dates of many of the pieces that were written prior to1784 had to be literally guessed at. Mozart’s own work catalogue was started by him in 1784, but even here a few works were not entered by Wolfgang and needed to be analyzed and assigned positions within the whole. Subsequent editions, especially the third by Alfred Einstein (K3, 1937), and the sixth by Franz Giegling, Gerd Sievers and Alexander Weinmann (K6, 1964, the most recent), included many corrections.

The Köchel catalogue is set-up to include the opening bars of each piece (if available, as some works are lost) and give each musical work a specific identification number, set in the (believed) order Mozart composed them in. These numbers, known as the K number, refer to Mozart’s works today; for example, the Symphony No. 41 in C major (the "Jupiter" symphony) is K.551. As well, there is a short description of the work and a listing of authoritative writings by others about the work in question.

Köchel also arranged Mozart's works into 24 categories that were used by Breitkopf & Härtel when they published the first complete edition of Mozart's works 1877-1905 in 50 sets (a venture funded in part initially by Köchel himself).

Since the first edition, there have been a total eight editions of Köchel's work issued. Only two others are significant: the third edition, revised by Alfred Einstein in 1936 (K3); and the sixth, edited in 1964 by Franz Giegling, Alexander Weinmann and Gerd Sievers (K6). The latter, especially, incorporated many newly discovered works and revised the dates assigned to many of the compositions previously.

In updating the catalog, the quandary editors faced was that Köchel's 1^st edition (now K1) numbers had become integrated into virtually every document related to Mozart, including books, newly printed score and all concert programs and recordings. So, even though modern research and scholarship could correct the dates of the many compositions requiring revision, "tradition" would not allow the catalog numbers to be merely rearranged. Though, in the end, a decision to do so early on would no doubt have eased the headache one associates with the present system.

The problem however, was "solved" by appending lowercase (and, later, uppercase) letters to the original numbers. This allowed new designations to be placed between the standard older ones. For one example, Mozart's Missa solemnis in c minor, "Waisenhausmesse", originally designated K.139 in K1, became K.114a (between K.114 and K.115) in Einstein's edition (K3), and K.47a (between K.47 and K.48) in K6. Obviously, this path provides a solution, but it can be confusing to newcomers as well as old hands. After all, it's much cleaner and simpler to refer to Mozart's "Paris" Symphony in D from 1778 as K.297 (K1) rather than as K.300a (K6). Also, don’t forget that K.417B is not the same work as K.417b, either.

Because of this, many modern publications have adopted one of two courses. The first is to follow Köchel's original scheme whenever possible and use later designations only for works that have been discovered (or reconstructed or re-evaluated) after 1862. The second path is to list after the title the main numbers in order that have been assigned to the work. So, the "Paris" Symphony might be seen as K.297/300a. Even then there are those Köchel works that have been assigned with four different numbers over the years. In such a case, publications generally use the identification numbers from the first Köchel version assigning a number, and then the latest K number.

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