Best comparison of Holmes Basic, B/X, and BECMI that I have seen

From RPG site forums

USC professor, occasional novelist, and (apparently) D&D fan Dr. J. Eric Holmes, as legend has it, took the OD&D rules (including Supplement I: Greyhawk), his own understanding of how some of the more cryptic parts were supposed to work, and possibly some common house-rules (it appears he must've had at least second-hand knowledge of "The Perrin Conventions" -- a set of combat house-rules used at Bay Area con games from 1976 on and eventually published in vol. 2 of Chaosium's All the World's Monsters), and decided on spec to re-edit the whole thing into something that made better sense and was more comprehensible to beginners, and then approached TSR with his manuscript and said "want to publish this?"

As this occurred right around the time D&D was beginning to really take off in popularity, and while Gary was immersed in is own attempt to compile OD&D+supplements into a more comprehensive and comprehensible game (AD&D) they took him up on his offer. Gary Gygax "revised" Dr. Holmes' manuscript and made a few changes -- mostly inserting plugs for the upcoming AD&D game, but also inserting a few tidbits from the working drafts (a couple spells, a couple monsters) -- and TSR released it as the DUNGEONS & DRAGONS Basic Game Set in the summer of 1977, both as a stand-alone book and in a boxed set together with copies of TSR's already-released Dungeon Geomorphs Set 1 (a set of blank maps), Monster & Treasure Assortment Set 1 (tables of pre-rolled encounters & treasure-lists) (Judges Guild hadn't yet proven to TSR the potential popularity of pre-designed dungeons, i.e. modules), and a set of dice.

Unsurprisingly, given its genesis, this was a strange set -- it includes some (but not all) of the new rules content from Supplement I (thieves are included, but only for humans (whereas SuppI allowed all races to be thieves); the convoluted chance-to-know spells by Int stat table is included for magic-users, but not the combat bonuses by Str table for fighters; variable attacks and damage are included for monsters (e.g. ghouls having 3 attacks for 1-3 damage each) but not characters (so all weapons do 1-6 damage regardless of size), and so on), its advancement tables only cover levels 1-3 but there are numerous unexplained references to higher-level characters and spells and a lot of included monsters (giants, dragons, vampires, trolls, etc.) that are way-overpowered for level 3 characters (not to mention the tables in the Monster & Treasure Assortment which contained numerous monsters, level 4+ character-types, and magic items not described in the rulebook), and the combat rules (which were largely absent from OD&D -- each individual referee was expected to cobble something together from Chainmail and his own common sense) don't bear much similarity to any TSR edition of D&D before or since (including anomalies like that each combat round lasts 10 seconds but 10 combat rounds equal a 10-minute exploration turn, that initiative order is determined by Dexterity score (which requires assigning Dex scores to every monster encountered), the fact that (even though all weapons do 1-6 damage) daggers attack twice a round and two-handed weapons attack once every 2 rounds, and that flaming oil is WAY better than any other attack form (doing 3d8 damage across 2 rounds)).

However, despite all this, and even after the AD&D books were released and had significantly-enough different rules from OD&D to render all those references Gygax had inserted incompatible, this set was popular enough to be reprinted about a half-dozen times over the next 2 years (including "upgrades" to the boxed-set version to include full modules -- first B1: In Search of the Unknown, later B2: The Keep on the Borderlands -- instead of the geomorphs & tables, and a downgrade when TSR ran out of dice sets and started including a sheet of "chits" instead).

Finally, in 1980, TSR decided to finally take the old OD&D whitebox (which had been sold as a legacy product, the "Original Collector's Edition," since 1977) out of print and replace it with a new, mass-market-friendlier edition (rumor has it TSR's legal settlement with Dave Arneson, who received co-author credit and royalties on D&D products but not on AD&D products, was also a factor in this decision -- that they were required to keep the former in-print, and distinct from the latter, to keep him from suing). This ended up being two sets, the Basic Set (a new revision of the Holmes set by Tom Moldvay) and the companion Expert Set (filling in all the gaps from the OD&D rules that weren't included in Basic, edited by David Cook and Steve Marsh).

Tom Moldvay's Basic Set is clearly based on the Holmes set -- it includes all of the same classes (including human-only thieves), most of the same spells (including Gygax's added AD&D spells like Remove Fear and (Tenser's) Floating Disc), more-or-less the same monsters and magic items (including those drawn from Supplement I like rust monsters, stirges, carrion crawlers, and gelatinous cubes that had no basis in pre-D&D literature or mythology) -- but the rules are considerably cleaned up and streamlined, with a lot of the weird little anomalies eliminated or made to fit more logical patterns. For the first time with Moldvay's set, the D&D rules actually worked and made sense as-written, without requiring each individual to essentially co-author the game.

The Cook/Marsh Expert Set was a direct sequel/companion to Moldvay's set that was also usable with the Holmes set (including a page of conversion notes at the front covering the main differences between the two), covering levels 4-14 (not sure why that particular number was chosen -- perhaps because it looked aesthetically pleasing on the book cover, or because that's where the thief's abilities max out in Supplement I) and including all of the "missing content" from the OD&D set -- the expanded advancement tables, higher level spells, more (and tougher) monsters and magic items, and the rules for wilderness adventuring (which had been completely glossed-over in the Holmes set) and castle-building (the original "endgame" for D&D) -- all slightly tweaked to fit and be compatible with Moldvay's revision of the system. Interestingly, nothing from Supplement I -- neither spells, nor monsters, nor magic items -- is included in the Expert set unless it had already been included in Holmes and Moldvay. So the Expert Set doesn't include, for instance, the Monster Summoning spells (or any spells above 6th level), the tougher Greyhawk monsters like ogre magi, umber hulks, and beholders, or anything from Greyhawk's extensive magic item lists (except for those couple of items -- bag of devouring and rod of cancellation -- that Holmes, and therefore Moldvay, had included).

These sets were also on the market for about 2-3 years (from late 1980 to mid 1983) and proved even more popular than the Holmes set (these were the "D&D fad" years, with 1982 apparently being TSR's all-time high water mark) and it was eventually decided to make an ever more beginner and mass-market friendly version of the game, and simultaneously "complete" it with the level 15+ rules that had been hinted at in the Expert Set (as the "D&D Companion") but never released (or, from what I understand, written). This job fell to Frank Mentzer, who had already had success overseeing TSR's line of choose-your-own-adventure "Endless Quest" books.

Mentzer's Basic Set is ruleswise almost identical to Moldvay's (there are a couple small differences, but nothing a casual or even moderately-dedicated player would notice) but radically different presentation-wise. Moldvay's book is organized as a straight-forward reference book; Mentzer's is a step-by-step instruction manual, including a (presumably Endless Quest-inspired) choose-your-own-adventure intro that takes up the first 20 pages of the book and lets the reader "play" D&D before actually presenting any of the rules. The result is that the same amount of rules now fill up double the page-count, and are much easier for beginners and young players (the boxed recommends ages 10 & up, down from Holmes' "adults ages 12 & up"), but are less convenient for in-game reference and have an authorial voice and tone that comes off as mildly condescending.

Mentzer's Expert Set is really just a reorganization of the same content as the Cook/Marsh version to better match the organization and look of the new Basic Set, and with some of the progressions slowed down at the upper levels to allow more "room at the top" for the planned Companion & Master sets. In addition to slowing down characters' saving throw advancement, thieves' skills, and cleric & magic-user spell acquisitions, a couple of the more powerful spells, monsters, and magic items are also held back and (IIRC) the castle-building rules are less detailed. Instead, we get a detailed sample base-town, and a couple pages discussing in-town adventures (something that OD&D had covered very briefly in a couple paragraphs and Cook & Marsh had ignored completely).

The D&D Companion Set (released in 1984, a year after the revised Basic & Expert Sets) was also written by Frank Mentzer, and includes both more complex optional rules (new sub-classes, new weapons with special effects (like nets and bolas), rules for wrestling and jousting) and extensive rules for higher-level (15-25) characters, focusing mostly on the establishment and administration of Dominions. There were also a set of abstract mass-combat rules, and some new, tough monsters. In retrospect this is where the line between mass-market-friendly D&D and hobbyist-oriented AD&D began to blur -- with the former becoming in some ways more detailed and complex than the latter (which never had an equivalent "High Level Campaigns" book in this era, and pretty much petered out after about 18th level) -- and the two lines began to look less like alternate approaches to the same general game and more like two distinct, competing games.

In the Holmes era, it was explicitly spelled out that you were supposed to play with the Holmes set until you hit level 3 (which was envisioned as taking about a dozen up to maybe 20 sessions) and then you'd switch over to AD&D. The Moldvay-Cook-Marsh sets don't actually say that (possibly because they couldn't because of their settlement with Arneson), but this still seemed to be the way they were actually used in practice -- start out with the Basic Set, play it up to level 3, get an Expert Set and play it for awhile, but eventually "graduate" to AD&D. I know plenty of people who played up to about level 6 or 8 (i.e. about 6-9 months of play) in Expert D&D before switching gears to AD&D, but I didn't know of anyone at the time who actually stuck with it all the way up to 14th level (which would, at the assumed advancement rates, take at least a year of play, possibly more like 2-3).

Thus, the Companion Set seemed weird, like it was saying that you weren't supposed to switch over to AD&D and were instead supposed to stick with this game for a multi-year campaign (which was reinforced even further with the release of the Master Set (covering levels 26-36) in 1985, and the Immortals Set (essentially a whole new game for characters who have "won" Master Set D&D and become immortal) in 1986 -- to actually get to use either of these in play would require several years of dedicated, regular play, which was (as we had always thought) what AD&D was supposed to be about, not the mass-market kid-friendly version).

I'm convinced TSR simply never thought this through, never considered that they were setting these games up not to have the mutually-beneficial symbiotic relationship they'd had in the Holmes and Moldvay days (start with the simple set, spend a few weeks to a few months playing through it and learning the game, then move on to the Advanced game which you can (at least theoretically) continue playing for years) but to be in competition against each other -- that by encouraging players to stick with D&D through the Companion, Master, and Immortal levels they were no longer adding to but taking away from sales of their flagship game.

But then, considering that this was the same era when TSR began introducing multiple different AD&D campaign settings and marketing each one as, essentially, a separate game -- that Dark Sun and Ravenloft and Dragonlance and the Forgotten Realms (and, ultimately, more than a half-dozen others) didn't really overlap and characters or adventures intended for one couldn't really be used in the others, meaning that casual fans would pick one favorite setting and ignore product branded for use with any of the others, thus hopelessly fragmenting the customer base and ensuring that nothing would achieve the kinds of sales levels that "universal" products and adventures saw back in the early 80s -- I suppose I shouldn't be too surprised...

(tl;dr version: the OD&D, Holmes, Moldvay, and Mentzer sets are all pretty close to the same, especially if you compare A to B, B to C, and C to D instead of A to D; TSR made a dumb mistake from 1984 on by ceasing to use the D&D line as a de-facto introduction to AD&D and instead trying to make it into a complete stand-alone (and thus competing against rather than feeding into AD&D) game)