This one was requested, so I accept no blame for it.
D&D is a cooperative game, as has been discussed, but it’s not really an even cooperation. The players in the party may all be equal, but the DM by design must have a leadership position. He or she is responsible for most of the session and plot planning, most of the party mood, much of the rulings on mechanics, and wearing any other number of hats necessary to keep a game running smoothly. For better or worse, a DM is basically in charge of the campaign, and the whole system is built with this in mind. Thus:
Law #1 – The DM is always right.
This is so core that d20 systems actually refer to it as Rule 0, which means it’s more important to the game than Rule 1 (which is “to determine the success of an action, roll a d20, add a modifier, and compare it to a target number”, or, you know, the d20 system). Everything that affects the game passes through the DM at some point, whether it’s deciding before the campaign which books are too powerful and which classes don’t fit in the settings, or deciding on-the-spot whether a creature counts as prone if they’re flying sideways ten feet above the fighter. Generally, the assumption is that the DM is the final arbiter of things happening at and beyond the table, trumping even basic rules and the results of the dice.
However, this can go too far. I’ve seen a DM ban rules, classes, feats, spells, races, and so forth based not on any necessity of the setting, game balance, or fun, but whether the DM owned the book, making the players’ books largely unusable and forcing that game into one person’s comfort zone. I’ve seen a DM change rules on the fly because he or she was upset that the players were surviving traps, not to increase tension but because he or she thought that there needed to be some punishment for triggering the traps at all. And at least once in every campaign, both as a player and as a DM, I’ve seen all play stop while a player argued with the DM about a ruling that went against the rules as written. In order to avoid situations like this, there has to be some limit on the DM’s authority, or at least the frequency and severity of leveraging this authority. Thus:
First Corollary – But not always entirely right.
A few years ago, I figured that the best way to approach this was to make a rules decision immediately, run with it for that encounter, and then entertain dissent afterward to determine a more proper way to handle a situation without bringing the live game to a halt. This has generally succeeded, including one memorable case where a player knocked a flying enemy prone. The enemy at the time wasn’t subject to gravity, so I couldn’t validate them plummeting to the ground, but I also didn’t think they should be immune to being prone. So I ruled the enemy “off-balance”, which was treated like being prone except that they maintained altitude. It worked, we finished the session, and nobody had a problem afterward, so we’ve used the same mechanic in other situations.
I’ve been overruled on a judgment, too. When I started in 4th Edition, I didn’t know that a creature at 0 hit points was considered to be dying. I was used to 3rd Edition, where they were staggered, capable of taking one action before collapsing. So in my early sessions, players and monsters at 0 hit points were dazed, but not dying until they hit -1. This was overturned quickly, but at least we didn’t spend ten minutes rifling through books trying to find the rules for hit points, which I could swear are in a different chapter in every edition.
I have some fairly experienced players now, so the biggest way this comes up these days is in hit points, which I track in the program I use to display session information. When a player disagrees with their hit point total, at least one of us is wrong. But too many sessions have gotten caught up in a recap of the player’s damage changes since the last time we agreed, which leaves a bunch of people sitting around watching two people do math. This is lousy, and I should be smart enough to avoid it, but I have a tendency to try to understand the situation (or make a player understand the situation) rather than just declaring one number or the other to be correct and moving on with the fun part. If a player really has a problem with my ruling on their hit points, I should be asking them to do a comparison every round or two just to make sure we’re on the same page and try to find out what’s missing, but I default to discussion. I’m not sure whether that’s good or bad, but it certainly isn’t quick.
I kind of went off on a tangent there. The point is that I don’t like Rule 0 being Rule 0, because it encourages DMs to make authoritarian decisions. It may be the first rule in the game, but isn’t not the be-all and end-all of D&D. A DM who doesn’t understand this is begging for unhappy players, so I arranged the order of my Laws with this in mind. It feels strange to demote something to merely being Law #1, but I think it’s a better description of what gaming should be rather than what gaming is.
For anybody interested, The Gamers: Dorkness Rising is at least partially about the conflict between Law #0 and Law #1 as the most important rule in a game. It helps that it’s a good movie.