As discussed previously, Dungeons & Dragons spread far beyond the wargaming audience that TSR initially aimed for. In Playing at the World, Jon Peterson speculates that D&D may have spread so because its setup didn't focus on 'war' in the sense of large-scale battles. The counterculture of the late 60s/early 70s was strongly anti-war, and, indeed, some within it went so far as to protest wargame conventions and wargames in general.
D&D did have combat, but it tended to be on a smaller scale, and the universe it posited, like that of Tolkein (who the counterculture of the time also seized on), featured a clear-cut morality that the members of the counterculture could no longer believe in with regard to conflicts in the real world.
There was also a hearkening back to 'simpler times' in much of the counterculture, and the medieval setting of D&D played to that. Of course, D&D wasn't the only thing that the counterculture seized on that did so - another was the nascent Society for Creative Anachronism, or SCA, which had initially formed in the late 1960s. Some of the SCA's founders styled it as a "protest against the 20th century", and the Society often describes itself as recreating the Middle Ages "the way it should have been".
Naturally, then, many of those in the already-existent SCA found themselves interested in D&D. These people were already practicing faux medieval combat with padded weapons and armor, and found D&D's wargame-based combat rules to be quite unlike their own experience.
Thus, players in the San Francisco Bay area began to create their own modifications to D&D combat. In 1976, Steve Perrin typed up and copied a set of such modifications, which he named "The Perrin Conventions", and distributed to the attendees at DunDraCon I, a D&D convention held in Berkeley.
Copies of the Conventions were borne back from DunDraCon to groups across the United States, and the Conventions were further spread by being included in the second volume of All the World's Monsters, an early 'universal supplement' for fantasy RPGs. Steve Perrin went on to be the primary creator of the rules for RuneQuest, which bears many obvious similarities to the Conventions.
So, what were the Conventions, exactly? Well, you can find them here, but these are what I consider to be the most interesting points:
1. The melee round was reduced from D&D's one minute to ten seconds. Similarly short melee rounds are seen in many later games - including Holmes' Basic D&D and its descendants.
(As a side note, the reduction to a ten-second melee round, with 'normal' movement being 120 feet per round, works out to a speed of about 8 miles per hour... which is also the same speed as D&D's standard outdoor running movement rate of 240 yards per 60-second round. This contrasts to D&D's own combat rules, where characters fighting indoors move at 1/3 the rate of those doing so outdoors because of the difference in scale.)
2. Rather than being randomly rolled, initiative was determined by Dexterity. There is, however, mention of the possibility of adding a random factor, and of allowing someone with a longer weapon to strike first against a foe with a shorter one. Dexterity could also be modified by armor, for both this purpose and other purposes. Holmes' Basic D&D also adopted this rule, although later versions of BD&D went back to traditional randomly-rolled initiative.
3. Characters who moved more than half their movement allowance were not allowed to make melee attacks. This is seen later on in Melee, and in the action system in the d20 System underlying recent versions of D&D.
4. The idea of using a "Dexterity roll" to determine success at actions such as changing a weapon, jumping over something, and so forth. Original D&D made no mention of the idea of directly using ability scores in this way (although Peterson's Playing at the World indicates that Dave Arneson's proto-D&D used ability scores for rolls... and also differed from the published D&D in many other significant ways).
5. Combatants could be knocked back or knocked down by blows, with an increasing probability as they lost hit points. These conditions each carried different penalties.
6. The Conventions had the idea of characters being "engaged" in combat (although not using that term). Withdrawing from combat with an opponent once engaged required an opposed dexterity roll, and carried the possibility that the opponent could get an attack in as you tried to break off.
These ideas were carried forward into many other RPGs. Steve Perrin says that he did not originate all of them, but gave the collection he created the name "The Perrin Conventions" because they were the ones he was choosing to use and was writing up together.
Overall, the modifications proposed by the Conventions make things much easier on GMs. A melee round which is a full minute long allows for a huge amount of movement in the course of it: presuming people can jog at about 8 mph (the "running" movement rate in classic D&D), they can cover more than the length of a football field in one round!
Such long rounds therefore make missile weapons much less useful than they are with shorter rounds: with the one-minute melee round, an attacker can move from being out of bowshot to melee range in one round. Further, since D&D without the Conventions doesn't prevent someone who has moved their full movement from attacking, they can then immediately attack! Depending on the GM, then, a warrior in non-Conventions D&D might close against a bowman and attack before a single arrow can be shot!
The Conventions change this greatly. The distance that the warrior closed in one round before (240 yards outdoors) now takes six rounds to close. This is the same 60 seconds of in-game time, but the bowman gets twelve shots off in that time! This makes missile weapons far more effective for defense. This in turn tends to make numbers more important. A dozen bow-wielding goblins become a much more significant threat!
The requirement to not move more than half your movement in order to make a melee attack also changes things a bit, and the concept of becoming engaged with a foe does it even more. All in all, these changes provide a GM with rules support to keep PCs from just running past foes, and enable some tactical considerations in movement. More limited movement makes it easier to form up lines to protect a target, makes formations such as combat wedges work better, and so on.
(This also makes horses actually useful in combat: in the standard D&D version, you can already close from out of bowshot to a bowman in one round. Unless you're facing opponents using catapults or other weaponry that can't be used by a single man, riding a horse provides no tactical advantage from speed. In the Conventions, however, a horseman can close from out of bowshot in 3 rounds instead of six, halving the number of chances the bowman gets to hit them!)
These movement restrictions also interact with the changed initiative system. While initiative is more fixed, the ability to gain first attack by preparing a missile weapon allows a tactical way around it. However, the Conventions also make it so that switching weapons is not "free"!
The addition of new combat states (being knocked back and knocked down) have significance as well. Knocking someone back or down allows disengaging from them with little risk, and provides a way to break up combat formations! They also make formations more important, by allowing for more situations where one character might need the help of another during combat.
Overall, these changes make combat much more tactical - there are many more significant decisions for players to make. While Tunnels & Trolls had actually abstracted combat even more than D&D, most subsequent fantasy games in the rest of the '70s would follow the lead of the Conventions toward more detailed combat.
In addition to the changes in combat, the shorter rounds mean that it's considerably easier to judge how much someone who's not engaged in combat can get done during a round. For example, in 60 seconds, I can walk through my house, not particularly hurrying, open the door to every room, and look into each and every one (not including closets). Pretty much no D&D GM would allow a party to check 9 rooms for opponents in one round! With ten-second rounds, allowing me to check one room per round would result in it taken 90 seconds... which is a bit longer than it actually took (58.65 seconds on my stopwatch), but not too bad. Thus, a shorter round makes it much easier for a GM to reasonably judge how much a character can get done.
We'll see the impact of the Conventions in our next game - The Fantasy Trip, which we'll be touching on in a few days!