The Thief

The thief, for all of its long history in D&D (having first been introduced in the Greyhawk supplement), is not the best-loved class among grognards. The most common complaint is that it "open[s] the door to a more generalized skill system, which I see as a large nail in the coffin of old school play," as James Maliszewski points out. Philotomy expands on this common objection:

Over time, I've come to prefer the game without the Thief class (i.e. using only the original three classes). The role the thief usually plays (scout/sneaky-guy) is easily filled by the other classes; everyone can attempt to be stealthy, search for traps, et cetera. Also, without the Thief and his special abilities, these activities are often performed by the player describing how he goes about it, rather than rolling against a skill, which I think is a lot of fun.

[Quoting Mike Mearls]:As others have mentioned, the thief is a self-justifying class. More importantly, I'd rather the players use critical thinking and deduction to figure out traps, unlock doors, and so on. I'd prefer to allow any player of sufficient creativity and wits to figure a way past an obstacle. To me, that's the appeal of original D&D.

I obviously don't have a problem with a generalized skills system, provided that it simply improves a character's chance of doing something, rather than restricing other characters' ability to make the attempt. However, the thief skills, having no parallel in the other classes, do pose a problem for the players conceptually if nothing else: If they look at the chart and see that their thief has only a 25% chance of disarming a trap (for example), they will naturally conclude that nobody else would have so much as a snowcone's chance in Gehenna. This is compounded by a statement in the Moldavy Basic rules that "[t]hieves are the only characters that can open locks and find traps without using magic" (p. 45, emphasis original).

But we needn't throw the baby out with the bathwater. There's a lot of appeal in playing a character who lacks magic or the strength to be a great warrior, but who lives by his wits and can simply sneak, perceive, climb, and make opportunistic attacks better than anyone else, like the Batman when he works with Superman, Wonder Woman, the Flash, etc. (And Batman is widely considered to be the most dangerous of the group.)

Here then is my own attempt to house-rule the thief's abilities in a fashion that melds with the skills system that I've created. Those who wish an "old school" thief that gets away from using the percentage die but which also does not presume any kind of systematized skills rules should check out James' house-ruled thief class. The following is designed to be compatible with Labyrinth Lords, OSRIC, and Swords & Wizardry (for which it was designed), and falls under the Open Game License:

The Thief

The thief is a man who lives by his wits rather than force of arms or eldritch might. This is not to say that the thief cannot fight when the need arises, but his real strength is the numerous skills he picks up in his travels. The thief has numerous skills that he brings to a party, especially stealth and detecting and bypassing traps and secret doors. Despite the name, not all thieves are out to steal; some are scouts in the employ of some lord, while others are simply rogues given to wanderlust, who travel the world to sate their curiosity. A thief is a welcome addition to the adventuring party, provided that he does not steal from his companions.

Hit Die Type: 1d6. After 9th level, a thief gains 1 hp per level (Constitution bonuses do not apply)
Armor/Shield Permitted: A thief can only wear light armor, and cannot use a shield.
Weapons Permitted: A thief may use any light blade, a longsword, a club, a quarterstaff, or any missile weapon.
Prime Attribute: Dexterity.
Special Abilities:
Backstab: When attacking an unaware opponent (e.g., after winning the surprise roll) or flanking an opponent with an ally, a thief may roll two dice for damage. At fifth level, he may roll three dice and keep the two highest results. At ninth level, he may roll four dice and keep the three highest results.

Extraordinary Climbing: A thief can climb sheer surfaces without the need for special equipment. His chances of success are 1-17 on 1D20. This chance increases to 1-18 at fifth level and 1-19 at ninth level.

Sneaky: Beginning at first level, a thief uses a d8 to surprise or sneak past an opponent when alone or operating with thieves of similar experience. Otherwise, a thief's surprise chance is equal to that of the least sneaky character in the group. This die increases to a d10 at 5th level and a d12 at 9th level. The thief may apply his Dex modifier when actively sneaking past.

Tools of the Trade: When possessed of appropriate tools, a thief can open locks and disable small mechanical traps on a roll of 6 or better on a d8. This increases to a d10 at fifth level and a d12 at ninth level. Note that especially well-crafted locks and traps may require a higher roll to successfully pick/disable.

Observant: At first level, a thief uses a d8 to detect secret doors and hear noises (needs a 6 or better in each case).

Wary: A thief can only be surprised on a roll of 6 or better on the surprise die.

Master Thief: A thief of 9th level or higher who constructs a hideout will attract 2d6 thieves of 1st level who will come to learn from him. Each of these thiefs will bring in 2d10 gp in guild dues and tithes a month, in addition to accepting missions from their master.

A quick note on the surprise die: In my house rules, a given person or group is surprised on a roll of five or better on the surprise die, not a roll of 1-2. This allows me to incorporate the surprise die right into my skills system.

Is it perfect? Probably not. Is it balanced and compatible with the ethos that a fighter should have a chance at hiding and stalking, or that a wizard should be able to try to climb that wall? I think so. Ultimately, we'll have to see it tested in-game.