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In Basic D&D, the default way to find a trap is to roll 1d6 and if you get 1 you find it. But thieves have a special % of finding it: e.g., a first level thief has 10%.

Should we roll this % passively? If not, should we roll both the d6 and the d% or just the d%?

From the manual:

Find or Remove Traps is a double ability. The thief has the listed chance of finding a trap (if there is one) and the same chance (if the trap is found) of removing it. Either attempt may only be tried once per trap.

While the default way is:

Any character has a 1 in 6 chance of finding a trap when searching for one in the correct area.

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As an aside, this article by one of the OSR elders is a fantastic read/interpretation of Thief skills in B/X:… – cr0m Jun 16 '14 at 18:17
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The reading of B/X leaves it open. In the period when it was for sale I can't remember a group that allowed a thief to roll twice. This is problematic at low levels because first and second level thieves have a lower probability of finding a trap than non-thieves. A 1 in 6 chance is 16.67% while first and second level thieves have a 10% and 15% chance respectively. For a character who is a specialist to be worse than non-specialists is problematic.

In the modern/OSR era the push for all characters to be able to try most things have revived the 1 in 6 which seems to have gone out of favor in the 3.x era (logically as it is handled by skills). In some cases these writers have argued that everyone has the 1 in 6 change to do thief skills (not just find/remove traps) but if a thief fails their skill is a second chance (or a saving throw). This is not the only or even most common interpretation from what I've seen but some OSR blogs champion it. One example is It's Okay, Gary Sent Us which is a revision of an earlier piece the author did for Fight On.

Doing some further research in my Dragon Archive CD, there is a Sage Advice answer in issue 62 about the chances of non-thieves having thieves skills and saying they do not. In general, the column was AD&D focused and without an explicit mention of B/X I don't consider it to overturn the 1 in 6 in an official manner. This point is emphasized by a question about contradictions between B/X and AD&D in issue 76 that said they were different and shouldn't be mixed. These were found searching Sage Advice in issues 52 (the introduction of B/X) through issue 80 (the end of 1983, the year Mentzer's boxed version of Basic came out). Based on the lack of an answer in Sage Advice in the relevant period I doubt TSR ever addressed the issue.

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Nice summary of the problem, the modern ambiguity, and the currently-popular community stance. I couldn't have said it better. – SevenSidedDie Jun 16 '14 at 17:11

The advice I give in this circumstance, and the method I use, is to allow level 1 and level 2 thieves to use the default 1-in-6 for detect, and the 10% or 15% for removal only.

The thing is, Moldvay's rules are pretty requisite on application of common sense. And common sense is that rolling to detect should be a single try, succeed or fail, so use the better chance.

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There are two sides to the issue: playability and verisimilitude.

In my experience, if you have a thief in the game they'll search every door for traps. It slows the game down. You might forestall this my not putting traps in the game, but then you nerf the thief by not letting him use a core ability. So you put traps in so the thief will have something to find, but now he needs to search everything. It's like a solution looking for a problem.

Back in the day, people handled this by establishing a "standard operating procedure" for doors and chests, along the lines of: check it for traps, poke it with a pole, listen for noise, all while standing back as far as possible. Since it was onerous to do this for every single door and chest, the thief would inform the DM upon entering the dungeon that he was "searching constantly" and that SOP would be followed at every door. So the end result was that the DM applied the thief abilities passively. I played in multiple groups that did this, it was the rule rather than the exception.

(The most extreme example was a group that used a cable and winch to stand back 50 ft and pull every door out of its frame...)

In terms of versimilitude, the thief has a worse chance to find traps than a non-thief until third level! That means having a low level thief in the party would actually reduce your chances to find traps. Also, it only stands to reason that anyone could attempt the same things the thief does. What makes the thief special is that he can do it better, faster, or or with fewer restrictions.

  • Anyone can open a lock, the thief can do it without a key.
  • Anyone can pick a pocket, the thief can do it without getting caught.
  • Anyone can move quietly, the thief can move silently like the undead.
  • Anyone can climb a tree, the thief can climb a sheer surface like a glass wall. Anyone can climb a wall with a rope, the thief can free-climb.
  • Anyone can hide behind a barrel, the thief can hide in plain sight if there are shadows.
  • Anyone can attack you from behind, the thief is more accurate and more deadly.
  • Anyone can find a non-magical trap (1/6) if they explicitly search, the thief can find magical traps even when not searching.

Some of these abilities verge on the mystical and that is a big part of what makes a thief special. But note that the thief's ability does not exclude doing it the mundane way. The thief can still use a key on a lock, use a ladder to scale a wall, or hide behind a barrel. What if he hides in shadows, behind a barrel? What if he tries to open it with key and fails (because it's not the right key) and then uses his open locks ability? He just made two attempts, one mundane and one skilled.

For these reasons, and because of the probability issue, I give the thief two attempts to find a trap: one by searching for one turn in the correct location, and one by using his special skill.

Regarding traps, there are several conditions mentioned for all characters: you have to take a full turn, you have to say you're searching, you have to be looking in the right place, the trap has to be non magical, and you only get one chance to find it. For the thief, it only says that you only get one chance to find it. The fact that one of the restrictions is repeated suggests that the other restrictions are not applicable.

That implies that the thief can find any trap, magical or otherwise, even when he does not say he is actively searching, and he finds it instantly. That essentially defines "passive detection".

So when DMing that is how I do it. I roll for the thief to detect traps passively, sort of a sixth sense for such things. If the party still wants to search for traps, the thief is welcome to join in and then gets another 1/6 chance to find it like everyone else. In this way the thief is always a benefit to overall searching ability, and has the additional benefit that he may detect something nobody expected. It speeds up play, and it makes the thief special and valuable.

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