In original Dungeons & Dragons, the three brown books only include one rule for traps. “Traps are usually sprung by a roll of a 1 or a 2 when any character passes over or by them.” That’s it. The rules never explain how characters can find traps without resorting to magic. This lone rule works with the early play style. If you wanted to find pit traps, you just told your dungeon master how you pushed down on the floor ahead with your 10’ pole. Or you sent your hireling ahead first.
In traditional D&D play, players rely almost entirely on their ingenuity to overcome traps and other obstacles in the game. Most players enjoy this style of play because their own observations, judgement, and decisions matter in the game world. If we preferred random chance and freedom from decisions, we would play Candyland.
In Book III, Underworld and Wilderness Adventures, Gary Gygax lists a dozen tricks and traps such as slanting passages, sinking rooms, and one-way doors. All foil mapping or freedom of movement, and none need rules to play, just player ingenuity.
Undoubtedly, Gary had thought of other traps such as spring blades, poison needles, and warning bells, but his list conspicuously omits any traps that seem to require game-world dexterity or knowledge to overcome.
In the May 1974 issue of a fanzine called the Great Plains Games Players Newsletter, Gary Gygax presented the thief class. In his introduction, Gary tells how the class was suggested by a gamer named Gary Schweitzer (probably Santa Monica gamer Gary Switzer). “He mentioned that his group was developing a new class of character—thieves. Gary gave me a few details of how they were considering this character type, and from from these I have constructed tentative rules for the class.” See Gygax’s “The Thief Addition” (1974) for more. In 1975, Supplement I: Greyhawk made the class official.
The thief class featured the ability to “remove small trap devices (such as poisoned needles)” At level 1, the thief boasts a 10% chance! So when your new thief says, “Don’t worry, I’ve got this,” the party should dive for cover. Original thieves enjoy no special ability to detect traps. Keep your hirelings in front.
The thief’s limit to disabling “small trap devices” seems to exist as an attempt to confine thieves to working on traps that require a character’s game-world knowledge and dexterity. For example, a chest rigged to release deadly gas requires a thief’s game-world finesse, and a die roll. Big traps like pits and rolling boulders, which can be beaten through player ingenuity, remain outside of the thief’s skills. Players can tell the DM the steps their characters take to bridge a pit or to chock the rolling-boulder trap.
In the summer of 1975, Gary Gygax brought the Tomb of Horrors to the Origins convention for a D&D tournament. One of the tournament’s players wrote a first-hand account of the event for issue 4 of the Alarums & Excursions fanzine. Even though the party includes two members of the new thief class, the Tomb offers virtually no place for them to disarm traps, and the Tomb [SPOILERS!] is loaded with traps. To determine when players get caught by traps, Gary fills the adventure with an ad-hoc system of saving throws, rolls of 1-2 on a d6, and verbal countdowns. (Player tip: If the DM begins to count down, run!) The Tomb’s legendary status comes from the mix of ingenuity, divination, and attrition required to bypass its memorable deathtraps, rather than the number of disarm checks needed. (DM Tip: if you run the Tomb and allow thieves to detect or disarm much, you’re doing it wrong. The Tomb of Tiresome Checks is a different adventure.)
In Advanced Dungeons & Dragons, thieves finally gained the ability to locate traps. A low-level rogue’s odds remained dismal, quickly upstaged when the priest gains Find Traps at level 3.
The rogue or thief’s limit to finding and disarming small traps remained in second edition. “These include poisoned needles, spring blades, deadly gasses, and warning bells,” but do not include “large, mechanical traps.”
In third edition, traps gained a systematic treatment, complete with triggers, effects, and difficulty classes.
By third edition, the trapfinding ability enabled rogues the chance to locate and disable anything that the DM categorizes as a trap, small or large, magical or mundane. This gave rogues more chances to shine, but heightened the tension between the traps a thief can find and disable and the traps that test player ingenuity. We have all encountered players who insist that a disable trap roll will enable their rogue to easily bypass some elaborate and cunning challenge. So does staying at home, but neither tactic leads to much fun.