The adjective ‘long-awaited’ is overused for academic publications, but entirely appropriate for the new version of the Köchel catalogue of Mozart’s works, which I first learned about in 1995 in conversation with its editor, Neal Zaslaw. Weighing in at a total of 1,263 pages, it is an imposing volume both in appearance – coming as it does in a robust red box – and in content, with individual entries on every work, lengthy appendices and a ninety-page bibliography, and including 125 pages of prefatory materials (in English and German) as well. It is also, by any measure, as momentous and historic an achievement as its illustrious predecessors in the catalogues by Ludwig Ritter von Köchel himself (1862), Paul Graf von Waldersee (1905), Alfred Einstein (1937), and Franz Giegling, Alexander Weinmann and Gerd Sievers (1964).

In the Preface, Zaslaw situates his new edition, styled KV²⁰²⁴, in the existing Köchel tradition. The original edition ‘represented a courageous attempt at a fine-grained chronology, which, however, was structured in a manner that rendered redating works, or adding newly discovered ones, impossible without inventing new numbers’ (xvii). Discoveries incorporated into subsequent editions compounded the problems, and have led to an important modification for KV²⁰²⁴:

All entries on individual works are newly written for KV²⁰²⁴ and attempt ‘to provide as complete, accurate, and up-to-date information as possible on all musical works’ (xix). Zaslaw’s notion of a ‘musical work’ by Mozart is also now understood ‘as a musical composition that has Wolfgang Amadé Mozart as its author and whose writing was begun with the intention of performance. This definition . . . differs from Köchel’s approach, which can be derived from the preface to the 1862 edition: For him, works in the narrower sense were only completed compositions’ (xl).

Each entry in the main sequence of works comprising the ‘Thematic Catalogue’ (‘Thematisches Verzeichnis’, 3–851) contains, when available and determinable, a great amount of information about dating, the autograph (including provenance), published facsimiles, sketches and drafts, early copies, arrangements, subsequent versions of the work, and important historical and modern editions. In addition, incipits are provided for all discrete movements and vocal numbers, and information is imparted about the texts of vocal and dramatic works; commentaries, brief bibliographies and references to published documents appear at the end. For long works like operas, entries often comprise around ten densely packed pages. I cannot sufficiently emphasize how useful it is to have all of this meticulously and fastidiously researched primary information in a single, comprehensive, immensely authoritative volume.

Individual entries, especially for early and less familiar works, are fascinating and enlightening, and enrich our understanding of Mozart and his musical world. The short entry on the sonata k10 composed in London (1764) is a case in point, where we learn of the existence of an authentic copy (the autograph is lost); of the dedication to Queen Charlotte that she apparently requested herself; of the unusual, elaborately designed dedicatory copy complete with fold-out pages to assist page turns; of the first edition only for keyboard and violin with the cello part printed separately; and of father Leopold’s decision to give the printing plates to London music publisher Robert Bremner before leaving London in 1765. Of course, this entry – like almost every other in the catalogue – builds on existing scholarship. It also gently corrects an error (about Thomas Wilkinson publishing the work) and acknowledges gaps in information (about its subsequent printing history), while communicating a remarkable amount in a single page (18–19). Such an entry will be useful for biographers, historians, students of late eighteenth-century musical culture and performers, as well as Mozart scholars. Valuable nuggets of information are even conveyed about short, ostensibly simple works for which it might have been assumed there is little left to learn, such as the motet ‘God is Our Refuge’, k20, written for deposit at the British Museum before the Mozart family’s departure from London (29–30). We are told that the manuscript is predominantly in nine-year-old Mozart’s hand (contrary to usual assumptions), probably resembling Leopold’s because Mozart followed his father’s prescribed template, and that the visit to the British Museum is recorded in the Europaeische Zeitung (1765), quoted by Georg von Nissen in his Biographie W. A. Mozarts (Leipzig: Breitkopf & Härtel, 1828). We also learn of the location of an early nineteenth-century copy and late nineteenth-century first edition.

The long entry for Mitridate, re di Ponto, k87, is a model of clarity and a treasure trove of information (105–114). We are alerted to the first setting of Vittorio Amedeo Cigna-Santi’s text, and its relationship with Jean-Baptiste Racine’s tragedy and two other opera librettos, as well as to the new texts for several numbers (probably by Giuseppe Parini, the house poet at the Regio Ducale in Milan, where Mitridate was premiered). The idea deriving from the secondary literature that Cigna-Santi turned to a translation of Racine’s tragedy by Parini is quietly corrected (106). The main characters and their creators at the original production are listed, autographs, sketches and drafts of individual numbers are described and explained (all with incipits), and many contemporary and later copies are cited, as are letters and documents in which Mitridate appears. Important factual information in the commentary, including the fact that Mozart left more than 150 lines of recitative text from the libretto unset as well as two entire arias and individual verses from several more, sits alongside an eminently reasonable interpretation that the large number of drafts of material for Mitridate bears witness to difficulties meeting singers’ demands. This is primary-source work of the highest order, inspiring those of us insufficiently familiar with an early opera seria in Mozart’s oeuvre to get to know it better.

Many later entries are equally insightful. Thamos, König in Ägypten, k345, is flagged in the Preface for its ‘chronological sprawl’ across more than a decade from 1773 (xix), and its various versions are masterfully explained in the work entry’s commentary (388–389). Commendably, moreover, there is no reluctance to identify known unknowns: ‘questions about whether Mozart was personally involved in the creation of the Lanassa version at all, and to what degree he was involved in which of the motets after Thamos cannot be definitely answered’ (xix). The unusually complex source situation for the ‘Paris’ Symphony, k297, including a third movement in the hand of a Parisian copyist rather than in Mozart’s, is also described in characteristically succinct and lucid fashion (327–330).

Any huge project compiled over several decades will leave some questions in the minds of readers. Occasionally, for example, commentaries go off on unnecessary tangents, or otherwise slightly astray. While usefully observing that the first edition of the Divertimento (Sonata) in D major for Piano Four Hands, k381, modifies one passage from the autograph in order to avoid a physical collision between performers, the commentary then pivots to discussion of the famous Mozart family portrait, where Mozart and sister Nannerl’s hands intertwine at the keyboard (439). As the commentary also acknowledges, though, it is doubtful whether the portrait depicts them playing an actual Mozart piece and that, even if it does, the notes indistinctly represented on the music stand apparently have no connection to k381. If the picture has no tangible link to the work – it was painted in 1780–1781, eight years after the composition of k381 – one wonders why it is brought up at all. Similarly, it seems redundant to mention the Polish count who loved a performance of the piano and wind quintet, k452, and subsequently commissioned a flute trio from Mozart (532–533), when the commentary doubts the veracity of the anecdote and gives us no reason to believe it. I wonder too about the rationale for suggesting Antonia Bernasconi as the recipient of the aria ‘Nehmt meinen Dank, ihr holden Gönner’, k383 (440), rather than the singer typically associated with it, Aloisia Lange, on the grounds that the poem addresses a woman bidding farewell to her patrons in order to seek her fortune abroad. (Bernasconi temporarily disappeared from the Burgtheater payroll in the spring and summer of 1782 shortly after the composition of the aria, albeit for reasons that are unclear.) Biographical links between life events and concert-aria texts were by no means standard practice for Mozart, and he much preferred the voice of Lange to that of Bernasconi, who he explained in late 1781 no longer possessed the ability to perform light music (a suitable characterization of the aria k383) in an effective fashion.

Eyebrows will also be raised for other reasons. While the Bibliography is extensive and contains more than 1,700 items, only around thirty were published after 2014, a quarter of which are by a single author, Ulrich Leisinger, who appears on the title-page via the Internationale Stiftung Mozarteum’s involvement in the Köchel project. The long and protracted genesis of KV²⁰²⁴ may offer a partial explanation, but such a paucity of recent scholarly items is a little disconcerting. On occasion, literature lists for individual work entries are too short and quirky: for example, the Piano Concerto in C minor, k491, a pivotal and much-discussed work in Mozart’s instrumental oeuvre, contains only four items, including a DMA thesis, a little-known edition and a publication from 1948 (589). And do we really need the alphabetical list of character roles in oratorios, musical plays and dramatic works? Even the Mozart aficionado itching to remember which works feature a Celia, Melia, Giacinta and Giuditta is more likely to search online than to pull this enormous volume off their bookshelf and turn to pages 1,117–1,118.

In addition to individual entries for every work up to k626 (the Requiem), the main thematic catalogue includes almost one hundred new Köchel numbers, from k627 to k721. (Discarding ‘chronological’ from the title of KV²⁰²⁴ renders it unproblematic to begin this new sequence with early Mozart works.) I am unconvinced that lost (‘verschollen’) works are deserving of a Köchel number in the main sequence, however, when their supposed one-time existence depends on information deriving not from Mozart, Leopold or wife Constanze, but more vaguely from someone else. The concerto movement k627, for example (795), depends entirely on Johann Andreas Schachtner’s testimony to Nannerl in 1792 about arriving at the Mozarts’ home with Leopold to witness the four-year-old Mozart at work on a concerto, and Leopold being moved to tears when realizing how good the concerto was. The story featured in Friedrich Schlichtegroll’s influential short biography (‘Den 5. December. Johannes Chrysostomus Wolfgang Gottlieb Mozart’, in Nekrolog auf das Jahr 1791 (Gotha: Justus Perthes, 1793)) and subsequently appeared in numerous publications, especially in the nineteenth century, thereby cementing its place in the biographical tradition. The Köchel commentary admits that the pre-1761 date is ‘probably inaccurate’ (‘wahrscheinlich unzutreffend’), but seemingly embraces the tale as reliably factual (795). Yet if the event’s date was in fact remembered incorrectly when Schachtner relayed his story around thirty years later, how can we be confident that its musical details are accurate enough to assign an unqualified Köchel number to the supposed work? I have similar doubts too about the presumed concerto for flute and orchestra k673, the cantata k678 and the aria k689, which are classified as ‘lost or not composed’ (‘verschollen oder nicht komponiert’). It is also strange that the Sanctus, Benedictus and Agnus Dei from the Requiem are only designated as ‘allegedly’ or ‘putatively’ (‘vermutlich’) by Franz Xaver Süssmayr (785), when they exist unambiguously in Süssmayr’s hand in the autograph. Clearly, the frequent scholarly speculation about Mozart’s possible involvement in these movements – unsupported by any extant source – has influenced KV²⁰²⁴ here. Such speculation, however, even when ingenious and provocative, cannot supersede philological evidence.

Time will tell whether the new Köchel numbers eventually seep into our musical consciousness alongside their illustrious predecessors. Given questions over the one-time existence of some of these compositions, I doubt they will. I also doubt whether the Köchel numbers of Mozart’s well-known arrangements, moved from the main thematic catalogue to the appendices (‘Anhänge’) of KV²⁰²⁴, will ever supplant their old numbers – including the five J. S. Bach fugues arranged for two violins, viola and bass, formerly k405 and now k Anh. A 46, and the Handel arrangements, including for Messiah, k572, now k Anh. A 57. As the new Köchel numbers in the Giegling, Weinmann and Sievers catalogue from 1964 demonstrated, scholars and the musical public have proved reluctant to change established habits and relearn ingrained facts based on new designations determined by new interpretations and scholarly orientations.

My small criticisms do not detract from a remarkable scholarly achievement and wonderful twenty-first-century monument to one of Western music’s greatest protagonists. It is highly unlikely that a thoroughly revised and reconceived edition similar to KV²⁰²⁴ will appear in future in hard copy, or even at all. Probably the best we can hope for is a periodically updated version of KV²⁰²⁴ online, similar to the New Grove Dictionary of Music and Musicians at Grove Music Online. The publisher Breitkopf & Härtel has thoughtfully provided not one but three ribbon bookmarkers for the volume, such is its size, scale and scope; copies situated on scholars’ bookshelves will also soon fill up with countless bits of paper and post-it notes, as reminders of invaluable bits of information it contains. Zaslaw is the first to acknowledge the team effort required for bringing such a huge project to fruition, not least Leisinger, Miriam Pfadt and Ioana Geanta at the Internationale Stiftung Mozarteum in Salzburg, many other scholars who have assisted him and several people at Breitkopf. Of course, it is Zaslaw himself who deserves most credit, though – above all for persisting valiantly with the project through thick and thin, and for more than three decades of the latter part of his career. He has made many outstanding contributions to the study of late eighteenth-century music and culture, but KV²⁰²⁴ will come to represent his greatest and most enduring scholarly legacy.