I'm still working on getting some actual plays up and I have a few other longer posts in the works that are sitting in drafts, but I'm going to hit on some quick thoughts in the coming days, the first of which is some thoughts on edition and how it's defined.
Dungeons & Dragons was first published in 1974 as a box set with three booklets. These weren't the normal Player's Handbook, Dungeon Masters Guide and Monster Manual that we're all used to, their organization was different, but I think the template was successful enough that it's stuck. If anyone is unfamiliar with this edition, let me tell you that it's a different creature altogether than the game you're used to. The layout is a little all over the place and the ties to wargaming are everywhere in the text. I'm veering off course already, but it really is a unique game and though supplements were printed for it, it stands alone as its own beast.
In 1977, two different versions of Dungeons & Dragons were published. Basic D&D, often called Holmes Basic since it was written by J. Eric Holmes, and Advanced Dungeons & Dragons. Basic released at the start of summer that year as a box set with dice and booklet or the standalone booklet, and featured rules that took characters to third level. As a further disconnect from the Advanced rules, Basic also featured race-as-class, so a player would just select Elf instead of Elf Fighter/Mage, which are the abilities the race featured. Advanced Dungeons & Dragons released later in the year and presented book releases with a format we've been familiar with since. AD&D also featured class/race combinations, multiclasses and most other trappings people associate with Dungeons & Dragons.
1981 saw a slight revision of the Basic Set featuring rewrites and edits by Tom Moldvay. This edit pushed Basic even further away from AD&D. This boxed set featured a red-bordered rulebook and module B2: The Keep on the Borderlands along with a set of dice. This revision was followed in short order by the release of the Expert Set, by Dave Cook and Steve Marsh. The Expert rules expanded character advancement from levels four to fourteen.
Two years later, in 1983, yet another revision of the Basic Set was released. Frank Mentzer's edit, also known as Red Box D&D, was followed in the next two years by four more boxed sets for the Expert (levels 4-14), Companion (levels 15-25), Master (levels 26-36), and Immortal (transcendence) rules. This release of the Basic rules were presented in a more tutorial form to welcome new players.
In 1989, Advanced Dungeons & Dragons got a 2nd Edition release, with updated rules and new character classes. This release was partially made to address the Satanic Panic that had impacted the game in previous years, removing 'evil' items like the Assassin class and Half-Orc race and changing the words demon and devil to tanar'ri and baatezu, respectively. The release schedule started over with the three core books and ballooned rapidly.
The Rules Cyclopedia released in 1991, collecting major rules from the previous Basic, Expert, Companion, and Master boxed sets. Aaron Alston and his team put together this tome as a reference guide for those with a higher proficiency with the game. In addition, the book included two new classes, a guide on conversion between D&D and AD&D and short overviews of both the Known World and Hollow World campaign settings (Mystara).
Also in 1991, a new Basic Set released that not only included a rule book, but a deck of Learning Cards that had rules of play on one side and an example of those rules on the back. The box also included a map, pawns, and dice. This box included rules that took characters to 5th level.
The final Basic Set release came out in 1994. The Classic Dungeons & Dragons Game, as it was called, was edited by Doug Stewart. The set came boxed with PC miniatures, monster pawns, a DM's screen, a map, and dice.
2000 brought a new edition of the main game line, dubbed Dungeons & Dragons 3rd Edition. This was a major overhaul of the rules that affected all aspects of the game, and introduced what was called the d20 System that led to the formation of the Open Game License. Notable among changes made were the removal of race/class restrictions and level restrictions for characters.
Three years later, a new release of the three core books revised the 3rd Edition, addressing plenty of minor rules complaints and confusions submitted to WotC. This revision, called 3.5, touched on races, classes, skills, feats, and almost every other aspect of the game. Supplements previously released were addressed in errata documents, but no rereleases and the publishing schedule retained its fast pace.
In 2008, another major overhaul of the primary game took place. 4th Edition Dungeons & Dragons introduced new core races and classes with a design that clearly pulled from video games with roles like tank and striker, enemy types like minion and elite, and powers with cooldowns and resets. Multiple waves of core books also made this edition notable. Mechanically, this edition deviated the most as each class had powers usable at different times and spells were rolled into this system, as well, making most classes feel similar in playstyle, but differentiating them with power sources and delivery. Every 10 levels (1-10, 11-20, 21-30) represents different themes of adventure; heroic adventurers are establishing their legends, paragon adventurers are affecting regions, and epic characters are world-shakers.
In 2010, a soft revision (though not called that) was released for 4th Edition, called the Essentials line. These books cleared up and revised some rulings while presenting the rules in an introductory fashion.
In the quickest turnaround for a full reworking of the main game line, Dungeons & Dragons 5th Edition was released in 2013. 5th Edition returned to many of the basic concepts of the d20 System familiar to players of the 3rd/3.5 Edition, though many rules come from all eras and editions of D&D. Defense values were boiled back down, a universal bonus was applied to skills, attack bonus, saves and other things characters are trained in. Advantage/disadvantage was introduced in a move to boil down granular situational bonuses that plagues previous editions.
Though this is presented as a timeline of release with a little bit of detail for each of the editions, my initial reason for this post is the argument in some groups that 3rd-5th Editions are not continuations of Advanced Dungeons & Dragons. Does that mean to imply that they're continuations of the Basic rules? There could be an argument for that, I suppose, as the Rules Cyclopedia states it's the second major revision of the BECMI rules, making it a 2nd Edition, of sorts.
Depending on source, there are 5, 14, 17, 18, 23, 28, etc., etc., etc. editions of Dungeons & Dragons. I've read all of those numbers in articles and blog posts, but I don't think I agree. I believe OD&D, the original 1974 release, is its own thing. I also believe BECMI is its own game and Rules Cyclopedia is included in that. Which leaves my absolutely reasonable (yet controversial) take that 3rd-5th Edition is the continuation of Advanced Dungeons & Dragons. The "Advanced" name could be dropped since the separate game was no longer in production or supported. This is also supported by the conversion document that showed how to convert your 2nd Edition character and other aspects of your campaign to the 3rd Edition of the game.