In OD&D, there’s a phase of the game that’s never mentioned in the rulebook, but still exists: the DM’s turn.
Remember that D&D’s direct predecessor was Chainmail, in which players alternated turns. D&D is a different type of game, but it might have taken Gygax and Arneson a little time to realize how different. Some traditional ideas, like opponents alternating turns, still linger. I think that if you imagine that the DM and the players alternate turns, it makes some troublesome terminology and some confusing passages make more sense.
In The Underworld and Wilderness Adventures:
Movement is in segments of approximately ten minutes. Thus it takes ten minutes to move approximately two moves – 120 feet for a fully-armored character. Two moves constitute a turn, except in flight/pursuit situations where the moves/turn will be doubled (and no mapping allowed) … At the end of every turn the referee will roll a six-sided die to see if a “wandering monster” has been encountered.
What? What’s a move, and why do you get two (or more) per turn? Why not just say, as does AD&D, that you move so far in a turn, and you check for wandering monsters after two turns?
And what about this passage about wilderness travel:
Turns: Each move will constitute one day. Each day is considered a turn. … Wandering monsters: At the end of each day (turn) the referee will check to see if a monster has been encountered.
Why does a day need to be considered a turn? Why not just call it a day? After all, “turn” is already the term for 10 minutes in the dungeon. Why redefine it?
It’s because D&D is a game, and a game needs turns. In a game, at the end of your turn, you cede control to the next player… in this case, the DM.
During the players’ turn, the players initiate all the actions. They open doors. They enter hexes. The DM can still react, of course, possibly with deadly effect. Traps might be sprung. An entire battle (at ten rounds to the turn), with the players and the DM’s monsters alternating actions, might take place, all during the players’ turn. But generally the players are walking around and messing with static monsters on a map.
At the end of the players’ turn, the DM gets to do some initiating. Both inside and outside, the term “turn” is defined by monster checks. In other words, after the players have a chance to move, the DM has the opportunity to introduce “wandering monsters” – moving monsters which force the players to react for a change. If we step back and think of D&D as a board game, and the DM and the players as adversaries (and many passages in OD&D suggest that they are!), we might imagine D&D as an asymetric game, sort of like Descent, in which the “Overlord” is explicitly give a turn and limited agency to play evil tricks on the players, or maybe like Dungeon World with its advancing evil fronts.
The “turn” becomes less and less important in later editions, but it’s emphasized several times in OD&D. And I can see how it can be useful. In later editions, the DM is often expected to play a pretty reactive roll. After designing the adventure, the DM sometimes does little more than run monsters and adjudicate traps and puzzles. But it’s useful to explicitly give the DM a turn every once in a while, after, say, ten minutes of dungeon exploration, or a day of overland travel, to check for wandering monsters; take a minute to think about what the bad guys are up to; or think of new challenges to throw at the players.