For combat rules players were directed to use Chainmail. This was a miniatures set but had a couple of tables at the back for fantasy use, cross referencing monsters and characters to see their chance of killing each other. The die rolls on the table were conducted using 2d6. For example, a Hero attacking a Troll might need to roll 9 or higher on 2d6 to kill the Troll.
Gamers who did not own Chainmail were required to use the "alternate" combat system included in the Men & Magic book. This alternate system is the combat system we are all familiar with today with its usage of 1d20 and cross-referencing the target's Armor Class. When the Dungeons & Dragons Basic Rulebook (First Edition) was released, the alternate combat system became the default, and no mention was made of Chainmail. Subsequent releases of the rules continued using the alternate combat system as the only combat system, relegating Chainmail's tables to the dustbin of history.
A very large reason for why the alternate system became the default was that the Chainmail system was rather limited in scope. For example, a +1 sword could double chances of hitting something at one end of the scale but make little to no difference in the middle.
Due to the Old School Renaissance (OSR) Movement and the rise of retroclones, sourcebooks and adventures that were not specifically put out by TSR are now grouped under Original D&D Compatible Products