Mention the names Mozart, Beethoven, Bach or Brahms to even non-classical music fans and they will almost assuredly ring a bell. But say Kchel, Hoboken, Longo or Deutsch and you’ll get a blank stare. Even classical listeners might have trouble recognizing them. Yet these are the names of gentlemen who are crucial to our knowledge and understanding of a musical genre replete with industrious composers who didn’t always keep the best records.

For generations of classical music lovers, a “K”-numbered work has come to mean Wolfgang Amadeus Mozart, but where did that “K” come from? It stands for educator and scientist Ludwig Kchel (1800-77), who took up residence in Salzburg, Austria (where Mozart had been born a century before). Here he made mineralogy indexes, categorized botanical specimens and discovered that Mozart’s works were in a state of chaos: The scores were undated and unnumbered, amounting to an undifferentiated mass of symphonies, operas, concertos, sonatas and so forth. How, Kchel wondered, could Mozart’s works be properly studied by posterity in this jumble?

They could not. Thus he undertook a heroic task: putting this repository of musical masterpieces in order, doing so by retracing Mozart’s steps throughout Europe and becoming an expert on Mozart’s handwriting by following the paper trail of correspondence he left behind. In 1862, Kchel produced a list of 626 works, from a minuet Mozart composed when he was 5 years old (K. 1) to the great Requiem mass he left unfinished when he died (K. 626). Kchel continued his work for the remainder of his life, collaborating with Johannes Brahms and others in eventually publishing a complete edition of Mozart’s works. Ironically, unlike Mozart’s infamously bleak passing, Kchel’s funeral was attended by royalty, accompanied by the music of K. 626.

In particular the earlier composers (17th-18th centuries) left many compositions to be authenticated, numbered and dated. The task was surely cut out for scholars who staked a claim to one or another composer whose works lay all over the European continent.

Austrian musicologist Otto Deutsch (1883-1967) worked on the sequence of Franz Schubert’s compositions (known by their “D” numbers), many of which were originally published out of order or never published at all. Anthony van Hoboken (1887-1983) was the Dutch musicologist who selected Joseph Haydn for his province (thus the “Hob” numbers). Alessandro Longo (1864-1945) was the Italian composer who created a near-comprehensive catalog of Domenico Scarlatti’s vast array of sonatas. But Massachusetts-born Ralph Kirkpatrick (1911-84) has given us the now-standard, authoritative Scarlatti numbering system (seen as “K” or, so as not to be confused with Kchel, “Kk”).

Ludwig van Beethoven and his successors normally published their works as they composed them (attaching an “opus number” to each), and in so doing posed few problems for the catalogers. Thus when Beethoven labeled his “Symphony No. 3 in E-Flat Major (Eroica), Op. 55,” we not only have the work’s genre (a symphony), its place among such works (third), its name (“Eroica” means “Heroic”) and its home key (E-flat major), but also its position in his total official output (his 55th).

Classical music is full of numbers and letters, which can be somewhat off-putting to the uninitiated. But the labors of Kchel, Kirkpatrick, Deutsch, Hoboken and others have actually made coming to grips with the classics far easier for everyone. They were quite often obsessed by the onerous tasks they took upon them, and for this we are eternally indebted to them.