B c'est Moldvay, X c'est Cook, BECM c'est Mentzer. OSE clone BX.
https://rpg.stackexchange.com/questions/2089/what-are-the-differences-between-holmes-moldvay-and-mentzer-dd a écrit : Differences between Moldvay and Mentzer
In general, the Mentzer version was a repackaging and expansion of the Moldvay version. The differences are minor in the Basic ruleset.
Moldvay aims his writing at a younger audience. Mentzer writes for an adult audience.
The Moldvay version has weaker layout and art than the later Mentzer version.
Moldvay's game is pretty simple and straightforward. Mentzer adds complexity, like skills and weapon mastery.
Moldvay's red box came with B2 Keep on the Borderlands, but you could buy the books separately (unboxed). Mentzer's Basic box didn't come with a module (the Expert set came with X1 The Isle of Dread, though!).
Moldvay stops at level 3 (and Cook's Expert set continues that to 14). Mentzer's "BECMI" continues on and on.
Moldvay states that clerics get their spells from gods. Mentzer gets all "wishy-washy"* about this aspect of the game and only says clerics get spells from "their beliefs."
Moldvay's magic-users get one spell at 1st level and have to find more in play. Mentzer's get one spell plus Read Magic at 1st level, and then one new one at every level.
The monster list changes between these versions. A number of people "monsters" are combined into the Human entry: acolyte, medium, trader, veteran. Insect swarm and noble are moved to the Expert rules. Some monsters are renamed: cave locust became locust, giant; driver ant became ant, giant; and killer bee became bee, giant.
Mentzer slows down the advancement of saving throws, thieves' abilities, and spell acquisition for clerics and magic-users.
Castle-building rules are more detailed in Moldvay, but Mentzer details a base town and talks about running town adventures.