In B/X D&D, PCs often try things that aren’t covered in the rules. If they come from later editions, they might expect there to be a pre-existing rule that says exactly what dice to roll and exactly what their chances are. There isn’t. Instead, the DM has to make something up on the spot. Here are some options for DMs, and some guidelines how to use them.
Mechanic #1: It Just Works
I can’t count the times I’ve had players say, “I want to do X, what do I roll?” when there are absolutely no stakes at all. But when all the characters have to lose is time, it’s a good idea just to let their plan work. The loss of time itself drains resources and imposes the risk of wandering monsters. That’s enough.
Mechanic #2: Flip a Coin (or something like it)
This is a recommended method from the Basic Rule Book (p. B60). You can just decide on odds of success and roll dice to see if the character succeeds.
This method works best for things that pretty much anyone, no matter their class or level, would have similar chances of succeeding at. It’s also best for situations that won’t come up a lot. The more you start trying to take into account various particularities of the situation and the person who is trying to accomplish something, the more you might need to look to other mechanics.
Mechanic #3: Ability Check
This mechanic, also described on B60, is similar in many ways to a skill check. In the roll-under style, a player rolls d20 (or 3d6) and compares the result to an ability score related to the task. They succeed if the result is less than or equal to the ability score. The target number might be modified as much as +/- 3 for difficulty. In the roll-over version, the target number is 21 – the relevant ability score, and it’s still a d20 roll.
This mechanic is best for cases when one of the abilities is clearly relevant to what the character is doing, but adventuring experience is not relevant. Characters with different abilities will have different success chances, but their class and level don’t influence their chances.
Mechanic #4: Reaction-like Rolls
This mechanic is similar to the ability check mechanic. The player rolls 2d6, with a modifier based on one of their ability scores. The modifiers might range from -3 to +3 or, in line with the way Charisma affects reaction rolls, -2 to +2.
You can extrapolate from the table for reaction rolls that there are 5 different results of a check like this. A generic version of the table might look like:
2: Failure, loss of resources.
3-5: Probable failure, loss of resources.
6-8: Neutral, but resources can be expended for further attempts.
9-11: Probably success, expenditure of resources can guarantee success.
12: Complete success.
This mechanic is best when it’s fairly clear what the five outcomes mean, character abilities matter, and class and level don’t matter.
Mechanic #5: Saving Throws
You can pick a saving throw that seems related to the task and have the player roll a saving throw of that type. If it’s especially easy or hard, you can apply a bonus or penalty to the roll.
This is, in some ways, the opposite of an ability check. It’s for tasks that any character of any class might be able to attempt and succeed at, but for which class and level are relevant to their chances of success.
I confess I don’t like this very popular mechanic very much. The differences in classes’ saving throws are not very systematic, and the saving throw categories are not organic at all. There are various rules of thumb people use for deciding which saving throw to apply, but I think the underlying question of why saving throws are being used at all is too-often ignored.
Mechanic #6: Attack Rolls
A lot of what people want to do is, ultimately, a matter of grabbing, striking, throwing, or aiming. For grabbing and striking, you can just set an AC and resolve it with a melee attack roll. For throwing or aiming, you can set an AC and resolve it with a missile attack roll. These rolls incorporate Strength or Dexterity, as well as class and level. It’s fine to use them.
This kind of mechanic is best for the tasks that are attack-like, in that they involve trying to affect a specific target, and either Strength or Dexterity is clearly relevant. The hard part is figuring out what AC to assign. But if you just keep in mind that a normal human hits AC 9 half the time, that’s a good starting place.
Mechanic #7: Opposed Rolls
This is really several different ways to handle cases when one character or monster is trying to do something and another character or monster is trying to stop them. When class, level, and toughness should matter, you can have each side roll their Hit Dice; high total wins. When those things don’t matter, you can have both sides make one of the other kinds of checks and let the “best success” win. For example, you could have opposed ability checks where the winner is whoever rolls highest without going over their ability score.
For that to work, though, both sides have to have ability scores, and monsters usually don’t.
The “opposed rolls” approach is best for things that are not attack-like, but where it’s fairly clear what each side should be rolling. In other cases, you might just pick one side or the other to be the side who has to roll and fall back on a different method.
A Final Note
Players cannot assume that everything they try will be resolved with a specific type of mechanic. This is good. It means they have to make decisions based on the world and the situation they are in, rather than based on the pre-determined bonuses on their character sheet.
But as the DM, it is your job to make sure the players are as clear about their odds of success as their characters would be. When they want to do something and it might fail, you must tell them as much as their character knows about their chances of success. That’s not just pointing out the consequences of failure, but actually giving them a sense of the odds. And don’t be vague about it. Don’t say, “It’ll be hard.” Say, “You’ll have a 1-in-6 chance of success.” Then there won’t be misunderstandings, and players can do better to manage their risks, make meaningful decisions, and have a fun game.
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