Cropped image from Wikipedia

Eric recently did a clarification post on thief skills for use with B/X D&D, based on the idea of leaving the basic percentages as is (that is, working with the basics of the traditional system without completely rewriting it). The thief class in my Pahvelorn game is a version of the Greyhawk thief, modified slightly to fit the mechanics of the 3 LBBs. My general approach is similar to Eric’s, though I have not committed to writing how the skills are resolved (despite much rumination on the thief class in general). Here is an attempt at guidelines for thief skill use.

Thief skills include Climb Walls, Hear Noise, Hide in Shadows, Move Silently, Open Locks, Pickpocket, and Remove Traps. All of these are percentile skills, other than Hear Noise.

The following general principles apply to all percentile thief skills.

1. Failing with a roll of 96, 97, 98, 99, or 00 means something goes wrong. The thief falls, the trap goes off, the lockpick breaks. Depending on the circumstances, the consequences of something going wrong may be dire (though a further saving throw might apply).
2. Failing with a roll of 95 or less means the thief makes no progress, but does not suffer any other negative effects. Another attempt may be made. Most attempts take one turn, but see below.
3. Succeeding by more than half means the skill use is quick. For example, if a thief has a 30% chance of success and rolls a 15 or less, the task was accomplished with alacrity. The exact amount of time required is up to the referee, but it should take much less than a full turn.

Move Silently is an attempt to approach or move past an enemy without being detected. Any action taken while moving silently automatically gains surprise.

Hide in Shadows allows a character to remain hidden even if someone is searching. Any action taken while hidden in shadows automatically gains surprise.

Regarding the two stealth skills, as specified above in principle 2, failing a Move Silently or Hide in Shadows roll does not mean that they are noticed (unless the roll is particularly bad, as laid out in principle 1). Neither Move Silently nor Hide in Shadows may be used in combat.

Characters other than thieves (or thieves that fail a stealth-related skill check) still have recourse to the standard chance of gaining surprise. This is 2 in 6 by default per encounter. It may be adjusted up or down based on the specific situation and character preparation.

Picking locks and removing small mechanical traps require tools and the special training of the thief. Larger traps must be disabled or avoided by player ingenuity. Traps may be discovered by using the same procedure for secret doors: 1 in 6 chance per turn (2 in 6 for demi-humans) given a 10′ x 10′ area, or by engagement with clues and explicit description.

Hear Noise functions exactly as the standard 1 in 6 listen at doors action, but with better chances.

Armor penalties apply to all percentile skills and are -20% if wearing chain and -30% if wearing plate. Hear Noise may not be attempted if wearing a helm.

I think this also reveals an interesting potential taxonomy. Listening and searching (the d6 checks) have to do with the state of the environment external to the thief. Whether or not a trap is present or a monster is beyond the door is not a function of the character. It’s either there or not. In contrast, all the other skills represent something about the character. Climbing, picking locks, removing traps, etc — these are all things that the character in some sense controls (particular task difficulty notwithstanding). Further, they are things that a character experientially can perceive the success of. They know whether they have failed to make progress climbing the wall or have fallen in a way that is different than a failure when attempting to use Hear Noise. Is there nothing there or did I just not hear it? This also explains why there is a Remove Traps percentage, but no Find Traps, as that is covered under the search action.

Thus, I propose the following final generalization: the d6 checks are rolled by the referee (to represent the objectivity and externality of the environment) and the percentile checks are rolled by the player. Even the Hide in Shadows and Move Silently skills, if looked at in the proper light, are not about being perceived by others, they are about the thief’s talent. Why shouldn’t the thief know whether or not they have successfully hidden in shadows? Thus, the thief can use the skill before they need to depend on it, unlike the surprise roll, which always happens when the thief is already potentially face to face with danger.

Note that this approach is more forgiving in several ways than the guidelines in Supplement I: Greyhawk, which specify (page 5):

The ability of a thief to climb is also a function of his level. There is a basic chance of 13% that a 1st level thief will slip and fall in climbing. With each higher level attained by the thief this chance is reduced by 1%, so that a 10th level thief has but a 4% chance of slipping.

And, regarding Open Locks, Remove Traps, Pickpocket, Move Silently, and Hide in Shadows (page 11):