The original version of Dungeons & Dragons did not have a thief class. It only had three classes: the Fighting Man, the Magic-User and the Cleric. They have expanded on their capabilities greatly over the years.
Gary Gygax introduced the thief in the first supplement, Greyhawk, which has very little to do with the actual World of Greyhawk and a lot to do with the extra rules he was using in his campaign at the time. (Blackmoor, on the other hand, has more in it of Dave Arneson’s campaign, although it is quite likely that the rules material did not originate with Arneson). However, before Gygax introduced the thief properly, he wrote up a version of it that was printed in a fan magazine. Jon Petersen has provided a copy of that publication on his excellent blog, Playing at the World.
One thing that struck me from the original presentation was that thieves automatically detected traps! A random percentage roll was made only to disarm the traps (the traps being set off on a failed roll). In addition, Gygax mentions that thieves are not intended to enter combat – with low hit dice, wielding a daggers and dressed only in leather armour, that’s enforced by their abilities, but that method of playing a thief has long fallen by the wayside, although it does explain a lot about some of their abilities in the early editions of the game.
It should be noted that the Greyhawk supplement doesn’t actually mention that thieves need to roll to detect traps; it’s something that I read into the material based on later usage. Eric Holmes follows the original D&D version of the thief, with the chances only of removing traps noted, but both Moldvay in the 1981 Basic Set and Gygax in the 1978 AD&D Players Handbook specifically note that a trap must first be found by a roll on the Find and Remove Traps table, and only once it could be found, could it then be removed.
A first level thief has horrible chances of removing traps. In AD&D, they only have a 20% chance of success, which is increased to 25% if an 18 Dexterity is possessed by the character – a rare event, indeed! (The original D&D gives them a mere 10% chance). Due to the double roll needed in AD&D, the thief’s chance of actually finding and removing a trap is only 4% at first level – given that the trap is likely to be a poison needle that will kill the thief, this means that most thieves, if they value their lives, don’t go anywhere near traps. Which immediately begs the question: if they’re also not meant to be in combat, what are they for, exactly?
It’s a question that D&D has been struggling with for a long time. D&D 3E improved their combat capabilities significantly; the change of how Dexterity applied to Armour Class aided them significantly, as did their new Sneak Attack ability. However, perhaps the most significant change of 3E came from the Take 20 mechanic, especially as it applies to Search, which gave thieves basically automatic detecting of traps, except those traps that were too difficult for a character of their level of expertise to find. It was the closest to how the original thief seems to have worked.
The new edition of the thief class keeps the separate rolls for detecting and removing traps, but the bounded accuracy of the system tends to keep those being too impossible. The thief remains pretty darn effective at combat, as well. The major problem that the new system has is in its incoherent approach to finding things. Does a thief use Perception, Investigation or Survival to find traps? (Consider that Survival is used to follow tracks, and that the animals you’d expect to be good trackers have Keen Senses… giving them bonuses on Perception checks but not Survival checks, and often don’t have Survival at all! Also, some of the adventures have the thief making Perception checks when actively searching for traps. At least a thief can succeed on the rolls. Which is a great improvement from the original D&D days.
Exactly how to deal with a thief finding traps is still a good question that is worth considering in your D&D campaign. One of the chief reasons for allowing thieves to automatically find traps is as a reward for playing a thief; something I think is definitely worth doing in the early editions of D&D, where the thief loses out in its combat abilities to the other classes. Perhaps automatic discovery when searching, and make the roll when they’re about to walk into something? The nature of traps that they can disarm has also changed over the years – from the small traps of poison needles to any sort of trap, including, possibly, magical ones.
For the time being, I’ll try the automatic spotting of traps in my AD&D campaign, while continuing with checks (and passive perception) for my 5E campaign.