OD&D’s initiative system is only barely described in the Three Little Brown Books, the assumption being that you have a copy of Chainmail to fill in the details. Many early players seemingly did not, and so initiative became a simple game of high roll wins. Supplements offered their own takes, included a segment-based option in Eldritch Wizardry, and AD&D famously has its own system of segments and weapon lengths and weird procedures when both sides tie, but simple side based or individual initiative systems won out in subsequent editions. A shame, really, as the way Chainmail handles it is pretty sleek and adds enough tactical considerations without slowing gameplay down.
How does it work? Every weapon has a Weapon Class (WC) based on length, ranging from a dagger at 1 to 12 for two-handed swords. A higher WC is advantageous during the first round of melee, as its reach allows for the first strike, but shorter weapons shine as combat continues and it is assumed that the participants close on each other. Weapons of similar length defer to initiative rolls and having the high ground overrides all other considerations. That’s it. Short, sweet, and surprisingly deep.
Coupling Chainmail’s melee initiative system with the idea of simultaneous action, makes things even faster and I would dare to say more realistic, or at least believable. Like Chainmail’s initiative system, simultaneous action is dead simple: everything happens at once, initiative is used to determine who has the edge when it matters. For example, if an archer declares they are shooting at a target who declares that they are diving for cover, initiative determines if the shot goes off before or after the target is in cover. If two sides charge each other, the initiative winner is considered the attacker, etc. Dead simple, easy to adjudicate, and fast.
The biggest hurdle to adopting this system is working out the Weapon Classes for monsters, but thanfully there’s prior art to draw upon, such as tables found in Seven Voyages of Zylarthen. I use a simple system that equates teeth and claws to daggers (1) and hand axes (2) and tails and limbs to maces (3) and flails (7), depending on creature size. As an added bonus, if you want to curb the power of high-level spells a little bit, you can use spell level as Weapon Class, patching the game in a similar manner to how it’s done in AD&D.
Anyway, that’s how Chainmail does initiative. One can only imagine how D&D would have evolved if this initiative system had been more widely adopted at the dawn of the hobby.