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.

  • \$\begingroup\$ As an aside, this article by one of the OSR elders is a fantastic read/interpretation of Thief skills in B/X: web.fisher.cx/robert/infogami/… \$\endgroup\$
    – cr0m
    Jun 16 '14 at 18:17
  • \$\begingroup\$ I will find you the answer from Mike Mornard if I can. Read it within the last year. \$\endgroup\$ Jun 28 '18 at 3:32

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.

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

There are two different trap mechanics presented in Moldvay's Basic D&D rules, the general one that uses 1d6, and the thieves' ability that uses d%. The reason for this, although it is far from obvious, is that the game distinguishes between two different types of traps. What Moldvay knew (but mostly failed to communicate) was that exactly one mechanic applies in any given situation, and which one applies depends entirely on whether the trap in question is a room trap or a treasure trap:

  • A room trap is a large architectural feature such as a concealed pit, or a block that falls from the ceiling when someone passes underneath. Indiana Jones movies are filled with of examples of this kind of trap. Room traps can sometimes be discovered by a thorough search of the area, taking great care and using common sense. Thus, any character who searches for them has a chance to find room traps. Dwarves, being most familiar with underground construction, have a 1 in 3 chance of finding them. All other classes, including thieves, have a 1 in 6 chance. The thieves' ability Find or Remove Traps does not apply to room traps.

  • A treasure trap is a devious little mechanism such as a poisoned needle hidden in a lock, or a vial of sleeping gas which shatters when a chest is opened. These kinds of traps are intricate and cunning. Only thieves possess the special knowledge and training needed to uncover treasure traps and disarm them. This expertise is reflected in the thieves' ability Find or Remove Traps. Only thieves are able to find or remove treasure traps without the aid of magic.

Note: The terms room trap and treasure trap are Moldvay's, which he coined on page B52. I find them a little misleading, but I have kept with them for the sake of consistency.


There are many passages in the text which support the notion that Moldvay saw a distinction between the kinds of traps that any adventurer can find and the kind that thieves alone can find and remove. Equally important, there are no passages in the Basic Rule Book which contradict the notion.

  • Page B22, in the section titled Traps

    Dungeons often contain traps, such as a trap door in the floor which springs open when a character walks over it...

    All the examples of traps given in this section fit the description of large room traps, and it says these are the kinds traps to which the 1d6 mechanic applies.

  • Page B9

    They [dwarves] are expert miners and are able to find slanting passages, traps, shifting walls, and new construction one-third of the time (a roll of 1 or 2 on 1d6) when looking for them.

    The references to mining, passages, construction, and so forth make it clear that by "traps" here he means large room traps not small mechanisms. The 1d6 mechanic is employed in this case, as expected.

  • Page B10

    They [thieves] are the only characters who can open locks and find traps without using magic to do so.

    On the surface, this statement seems to contradict the rule that any character has at least a 1 in 6 chance of finding traps. However, Moldvay is referring specifically to the thieves' ability to find treasure traps and so there is no contradiction. Thieves really are the only ones who can find those.

  • Page B10

    A thief's training includes learning how to pick pockets, climb steep surfaces, move silently, hide in shadows, open locks (with a set of lockpicks or burglar's tools), remove small traps (such as poisoned needles), and how to hear noises better than other humans.

    Here he is going through each of the seven standard "Thieves' Abilities". When he comes to traps, he only mentions removing "small traps" and even gives an example of just what he means by that.

  • Page B19

    It also takes a turn for a character to search a 10'x10' area, for a thief to check an item for traps, to rest or to load a bag with treasure.

    Note that he gives the specific example of a thief checking an item for traps, and this activity is treated as distinct from searching an area.

  • Page B52

    A Trap may be located in an empty room or on treasure.

    Moldvay calls these two types of traps "room traps" and "treasure traps" and lists some examples of each. At this point in the rules he is describing a system for randomly stocking dungeon rooms. It could be a coincidence that he took this either-or approach. On the other hand, this seems like just the approach he would take if he felt it important that a DM who runs a randomly stocked dungeon should know the type of each trap in order to know which mechanic applies to it.

  • Pages B55-B56

    The example of populating Room 3 has the DM determining that the room should contain treasure with a trap. Further down the page, the final description of Room #3 has this:

    The safe is trapped with sleep gas which will billow out unless the trap is deactivated.

    This indeed sounds like a treasure trap and so only a thief should be able to do anything about it. The fact that he says it can be deactivated (without explaining how) suggests that he's assuming a thief would use his Remove Traps ability.

  • Page B59

    Silverleaf is checking for secret doors, Fred is looking for traps, Black Dougal is examining the box, and Sister Rebecca is guarding the door.

    I believe that here Moldvay is giving us an example of a party using the different trap mechanics to best effect. Fred (Fredrik) is a dwarf and with his 1 in 3 chance is the one best suited to search for room traps. Black Dougal is a thief and the only one suited to search for treasure traps.

    It later notes that the DM rolls for Dougal's "Find or Remove Traps" ability while he examines the box. According to the "Thieves' Abilities" table, being 2nd level gives him a 15% chance of success. The 1 in 6 room trap mechanic would have given him a 16.6% chance of success, if it applied. Certainly Dougal would insist that the DM use the mechanic which has the greatest chance to succeed. But the fictional DM didn't use the 1d6 mechanic, the reason being that he couldn't. Dougal is examining an item for traps, not an area, so only the thief ability applies.

  • Page B21, in the section on LISTENING

    LISTENING: A character may try to listen at a door to hear what is on the other side. For each character listening, the DM should roll 1d6. A result of 1 (1 or 2 for demi-humans) means that the character hears any noises being made by the creatures on the other side of the door (if any). [...] Thieves have special chances to "hear noise" (see page B8).

    Although this one does not relate directly to traps, it does show us how Moldvay handled a situation where he saw an overlap between a thieves' ability and something any class can do.

    Any class can listen at doors, and the mechanic for this uses 1d6. Thieves have their own chance of success which improves as they go up in level. But unlike other thieves' abilities which use d%, Hear Noise uses 1d6. Moldvay saw an overlap between the thieves' ability and the general one and so he unified the mechanics.

    The fact that Moldvay did not similarly unify the mechanics for finding traps lends further support to the notion that he didn't see any overlap between the thieves' ability and the general one.

  • AD&D Players Handbook, Page 27

    Finding/removing traps pertains to relatively small mechanical devices such as poisoned needles, spring blades, and the like. Finding is accomplished by inspection, and they are nullified by mechanical removal or by being rendered harmless.

    Although this one is not from the Basic rules, it does seem acutely relevant. The PHB predated Moldvay's rules by three years. Could it be that when the Basic rules were written this detail about the thieves' ability was already so well understood as to be assumed, so that neither Moldvay nor his proofreaders ever noticed that he failed to state it explicitly?

  • Palace of the Silver Princess (Module B3) by Tom Moldvay, Entry 53, Page 12

    The DM should roll 1d6 for each character looking for a trap. On a roll of 1 (1 or 2 for dwarves) the character will discover a trap, unless the character is a thief. The chance to find a trap for a thief, is equal to their "Find Traps" special ability. Roll percentage dice for each thief looking for a trap. First level thieves have a 10% chance of finding a trap. Second level thieves have a 15% chance of finding a trap. Third level thieves have a 20% chance of finding a trap.

    Now let's not all start hyperventilating. Although Moldvay is credited as author of the module, he was not necessarily the author this particular passage. Module B3 had a complicated history (read the Wikipedia article). It credits several people with "Development" in addition to Moldvay and he is not listed under "Editing". Furthermore, the awkward structure and multiple grammatical errors in this passage suggest to me at least the possibility that someone other than Moldvay could have written it. And let's not overlook that the rule itself simply beggars common sense! That venerable Moldvay wrote those words with full cognizance seems unfathomable to me. Yet there they are, and his name on the cover. Each man must do bloody battle with the fact in the coliseum of his own mind.

    Grudging thanks to Kaique de Oliveira for bringing to light this intriguing, if inconvenient, piece of the puzzle.


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.


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.


I really liked this toom/treasure trap classification to solve this problem. But I don't believe it was something Moldvay was considering as part of the rules. There's a passage in the Palace of the Silver Princess (green cover) where Moldvay clearly states:

Page 12 - Entry 53

"The DM should roll ld6 for each character looking for a trap. On a roll of 1 (1 or 2 for dwarves) the character will discover a trap, unless the character is a thief. The chance to find a trap for a thief, is equal to their “Find Traps” special ability. Roll percentage dice for each thief looking for a trap. First level thieves have a 10% chance of finding a trap. Second level thieves have a 15% chance of finding a trap. Third level thieves have a 20% chance of finding a trap."

I guess it was just a design flaw.

  • \$\begingroup\$ Moldvay may not have written the passage you quote. Several people are credited with "Development" of that module. Also, the style of the passage strikes me as a bit sloppy for Moldvay. And consider the following from PHB (1978, three years before Basic): "Finding/removing traps pertains to relatively small mechanical devices such as poisoned needles, spring blades, and the like. Finding is accomplished by inspection, and they are nullified by mechanical removal or by being rendered harmless." Room/treasure trap classification seems reasonable whether Moldvay wrote the above passage or not. \$\endgroup\$
    – Fatootie
    Jun 28 '18 at 2:06

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