One must suppose, if we are going to make sense of Haydn's rather extensive oeuvre, all we need to do is figure out the system used by the main cataloger and it's all downhill from there. Oh, you of too much faith. Surely you know, nothing Haydnish is ever quite so easy!
Anthony von Hoboken was from Rotterdam, Holland, and he came to music from an engineering background, which gave him a necessary foundation for organizing Haydn's works; “First we clear the swamp!” Since I refer constantly in these essays to Hoboken numbers, let’s get an idea how the engineer solved the problems.
Haydn wrote in nearly every genre of his time, and even today a precise chronology of the entire oeuvre is non-existent, so Hoboken was faced with a momentous task of organization. Unlike your more usual catalog (like the quasi-chronological Mozart/Köchel) Hoboken decided to divide his into genres. He gave each genre a Roman numeral, and each work within it an Arabic numeral. What could be simpler? Oh, did I mention this? With only a few exceptions, the chronology Hoboken imposed on the secondary, Arabic numerals is not accurate? Where would the sport be if this were the case?
Let’s look at the list of genres. The numbers in parentheses are the number of works currently included in each category, but we must keep this in mind; not all the works that Hoboken listed are authentic, so some have been deleted since they were first entered, leaving a gap in the numbering. He was amazingly accurate though, given what he had to work with;
I Symphonies (108)
Ia Overtures (16)
II Divertimenti for 4 and more Instruments (47)
III String Quartets (83)
IV Divertimenti for 3 Instruments (not including keyboard or Baryton) (11)
V String Trios (21)
VI Various Duos (6)
VII Concertos for Various Instruments
VIII Marches (7)
IX Dances (29)
X Various Works for Baryton (12)
XI Trios for Baryton, Violin or Viola and Cello (126)
XII Duos with Baryton (25)
XIII Concertos for Baryton (3) (all lost)
XIV Divertimenti with Piano (13)
XV Trios for Keyboard, Violin or Flute and Cello (40)
XVa Keyboard Duos (Violin sonatas) (1)
XVI Keyboard Sonatas (52)
XVII Keyboard Pieces (12)
XVIIa Keyboard 4 Hands (2)
XVIII Keyboard Concertos (11)
XIX Pieces for Mechanical Clock (Flötenuhr) (32)
XX Works about The Seven Last Words of Christ (2)
XXbis Stabat Mater
XXI Oratorios (3)
XXII Masses (14)
XXIII Other Sacred Works (24)
XXIV Cantatas and Arias with Orchestra (including 'Insertion Arias') (29)
XXV Songs with 2, 3, and 4 Parts (16)
XXVI Songs and Cantatas with Piano (50)
XXVII Canons (Sacred 10; Secular 47)
XXVIII Italian Operas (13)
XXIX Marionette Operas (Singspielen) (7)
XXX Incidental Music (3)
XXXI Arrangement of Scottish (273) and Welsh (60) Folk songs (334)
All told, a tidy sum! So what sorts of oddities might one encounter when reading a Hoboken listing for a work? I will attempt to list here the ones I have encountered. Note here; For my own personal use, I never use the Roman numerals, it is just easier for me all around not to. Neither do I spell out the words 'Major' or 'minor'. I use lower case for minor and uppercase for major. Also, in order to facilitate reading across computer platforms, I use an italic, lower case 'b' as a stand-in for the 'flat' symbol. So if I let something slip by, don't be confused, it's just a peccadillo of mine. Hopefully these examples will answer most of your questions.
Some sample listings;
A normal, standard listing;
Hob. IV:1 Divertimento in C (Major) for 2 Flutes & Cello
Hoboken Group 4 (Divertimenti for 3 instruments), number 1.
Rather straightforward. Wish they were all like this. Of course, No. 1 was composed at nearly the end of his career, but we won't go there for now.
A concerto (not for keyboard);
Hob. VIIe:1 Concerto in Eb (flat Major) for Keyed Trumpet & Orchestra
Hoboken Group 7 (Concertos for Various Instruments) subgroup 'e' (trumpet concertos), number 1 (and only).
Hob. VIIa:3 Concerto in A for Violin & Orchestra
Hoboken Group 7 subgroup 'a' (violin concertos), number 3.
See how easy this is?
Now a more difficult one, since even knowing the meaning doesn't necessarily make you completely happy.
Hob II:G1 Divertimento in G for 2 Oboes, 2 Horns, 2 Violins, 2 Violas & Basso
Hoboken Group 2 (Divertimenti for 4 or more Instruments) the 1st listing in G Major. You will see this sort of thing many times if you collect Haydn recordings. The easiest way to remember it is thus; a letter on the right side of the full-stop or colon always indicates the key the work is written in. The reason it doesn't have a regular Hob. number is because at the time the catalog was compiled, this work was only attributed to Haydn. No one knew for certain whether he had actually composed it or whether one of the legion of unscrupulous publishers of the time was trying to fob off a work already on the shelves by tying it to Haydn's very salable name. If it now occurs in a list of authentic Haydn works, such as the one used here, it means since the time it was cataloged, evidence has been discovered which makes it virtually certain Haydn really did compose it. I chose this particular example with a purpose, actually, so I could make this point for you; only two of hundreds of works in Hoboken II have actually been authenticated, the aforementioned G1 and this one;
Hob II:D22 Divertimento in D for 4 Horns, Violin, Viola & Basso
Which yes, you interpreted correctly if you said the twenty-second listing in D major. There are plenty more where this one came from though, since D is the most popular key for divertimenti. I own recordings of a great many works listed as 'Hob. II', and as far as can be documented, only those two of all the 'attributions' are actually determined to be authentic. Many of them are nice music. In some cases, we actually know who wrote them. We aren't even sure Haydn didn't write some of them. There is simply no telling at this point in time who did for sure.
One more Hoboken thing...
A last thing we will see is the occasional use of the German deest. It simply means 'not appearing here' (or, in context, 'not appearing in Hoboken'), and indicates a work of which any association with Haydn was unknown when Hoboken wrote his catalog. I am unaware so far of anyone having the intestinal fortitude to actually assign a new Hoboken number to an authenticated deest work, or for that matter, to a work such as I discussed above, which has been authenticated and should be moved to a real number. The fact is, the entire catalog is in drastic need of a touch-up. Whether the inbred conservatism of the academic world will ever allow this to happen remains to be seen. But for we who listen to and collect Haydn recordings, or are merely trying to wade through the occasional musicology text, the lack of consistent order has become something we tolerate because the status quo is so entrenched. This being the case, we might as well learn what we can and make the most of it!