In the original Dungeons & Dragons 3-Volume Set, the standard combat that became the default in later editions of the game was explicitly an optional alternative combat system. The default was to use the rules Chainmail. However, the alternate system effectively became the default, since wasn't clear, even to those readers familiar with Chainmail, how exactly that was supposed to work.

Using Chainmail to Resolve OD&D Combats is an attempt to clarify how the two rulesets can work together. However, it's only one interpretation; other interpretations are possible.