With the announcement of the 2024 revision of 5e, we've had another go around of the Edition Wars kicking off, and one of the major battle grounds is simply the question of what constitutes an "edition". How many editions of D&D have their been, anyway?
The answer is astonishingly complex, due to changes in publisher, inconsistent use of the term "edition", and legal fictions. Plus, it's a really touchy subject for some people. So, let's have a look.
TSR Era
For legal reasons, TSR maintained the legal fiction that they published two completely separate games: the "Dungeons & Dragons" game created by Gygax and Arneson, and "Advanced Dungeons & Dragons" created by Gygax alone. To a large extent that was a nonsense, of course, but it's something we have to deal with.
Dungeons & Dragons
There are four pretty clear delineations of D&D versions under TSR: OD&D (or 0e), Holmes basic, B/X, and BECMI. Adding one more wrinkle, the BECMI line culminated in the rules being republished in a new format: the "black box" and "Rules Cyclopedia". These didn't contain any rules changes, other than the incorporation of minor errata.
Advanced Dungeons & Dragons
The line here is fairly clear: there was 1st Edition and 2nd Edition.
The only wrinkle was that during the life of each edition TSR reissed the core rulebooks. In 1st Edition, the change amounted to new covers, with the interior being unchanged (except for minor errata), while in 2nd Edition there was an entirely new layout but no rules changes (again, except minor errata).
WotC Era
When Wizards of the Coast took over, they made final settlement with Dave Arneson, and consolidated everything back to one game: "Dungeons & Dragons". Since then, there have been five publishing 'events' of note:
3e
This one is pretty straightforward - a new edition that was entirely incompatible with the past, even moreso than at any time in the D&D/AD&D split.
3.5e
A revision that made comparitively minor changes to an awful lot of rules. Given the precedents of 1st -> 2nd Edition, this should have been considered a new edition, but WotC very clearly stated that it was no such thing.
4e (2008)
Again, this one is pretty straightforward - a new edition that was entirely incompatible with the past.
The one wrinkle with 4e, which will become important later, is that this edition saw much heavier use of "errata" than previously, to the point where the word 'errata' was being used incorrectly - a lot of the changes were outright revision. (Incidentally, that misuse of 'errata' began in 3.5e when they made a significant revision to the rules for polymorph, and continues in 5e. It was particularly egregious in 4e, but by no means confined to that edition.)
4e Essentials (2010)
This is where it gets really controversial. WotC put out a new set of rule books that coincided with the release of a major set of errata/revisions. However, these were backwards compatible in a way that 3.5e was not, meaning that they amounted to a set of new options for 4e play.
Once again, WotC were clear that this was not a new edition, with much more justification than in the case of 3.5e.
5e
Another simple one - a new edition that was entirely incompatible with the past.
So, How Many Were There?
The answer to that is that it depends on how you count:
Using the IP Owner Definition
If we accept that the IP owner of the time knew best, then the answer is pretty clear: 4 versions of D&D and 2 of AD&D under TSR, plus 3 editions under WotC: 9 in all.
Release of Replacement Core Rulebooks
If instead we assume a new edition comes with the release of new core rulebooks, and where the choice of which version to get matters (that is, excluding 1st Ed's "new cover" books - the complete lack of interior change means it doesn't matter a jot), then the answer is 13: 5 versions of D&D (including BB/RC) and 3 of AD&D (including the "black cover" 2nd Ed) under TSR, plus 5 versions under WotC. (Essentials is counted in this one, because a new player should rightly be guided to the new books.)
Rules Incompatibility
If we define rules incompatibility as "I probably need to update my character sheet", then the "Rules Cyclopedia" and "black cover" 2nd Ed books are excluded, as is Essentials. However the transition from 3e to 3.5e does count.
Under this measure there are 4 versions of D&D and 2 of AD&D under TSR, plus 4 versions of D&D under WotC: 10 in all.
What do I think?
My answer is 10 - I'm inclined to follow the "rules incompatibility" scheme, and think that 3.5e is incompatible enough to count as a new edition, but the move to Essentials does not. (I find the "it's just new options" argument compelling - that said, I detest the misuse of the word 'errata' that was, and still is, applied.) And neither the "Rules Cyclopedia" nor the "black cover" 2nd Edition books count.
And the 2024 Revision?
At this point it's hard to tell. It looks very much like the changes are much smaller than from 3e -> 4e, and smaller even than those from 3e -> 3.5e. However, they do look more extensive than those from 4e Classic to 4e Essentials.
So, this will clearly not be a new edition under the "IP Owner" scheme, will be a new edition under the "New Rulebooks" definition, and might be under the "Rules Incompatibility" scheme (which is the one I'm using).
I guess we'll see!