Here are documented interpretations and explanations of the thief skills; some are full-on houserules, whereas some are simply explanations of the way the rules are adjudicated. Apart from the houserules listed here (such as the detection rules for picking pockets), this page exists mostly as a "cheat sheet" to clear up the common misconceptions about the way thief skills work.
Much of the Complete Book of Thieves is allowed. The following NWPs are permitted: Alertness, Begging, Boating, Fast-Talking, Fortune-Telling, Intimidation, Locksmithing, Looting, Observation, Trailing, and Voice Mimicry. Except in rare circumstances determined by the DM, these NWPs are only granted to thieves and assassins. Many of the specialised equipment is found in the books is found amongst professional thieves or guilds, as well. Specifically, the Arm Sling and Mini-Blade (pickpocketing); Acid, Chisels, Magnifying Lens, and the Oil and Funnel (lockpicking); Weaponblack and Charcoal (hiding); Special Function Arrows and the Spike-and-Line (climbing); and miscellaneous tools such as the Glass Cutter, Housebreaker's Harness, Keymaker's Set and Wax Pads, Limewood Strips, and Tar Paper. Finally, all of the Evasions listed in the book exist to various degrees of availability.
A popular thief weapon that is not listed is the blackjack. The blackjack has a value of 6 gold pieces, and is identical in every way to a club with two key difference. Half of the damage dealt by a blackjack is always nonlethal damage (with no penalty to hit), and it is possible to get a sneak attack with a blackjack.
A roll of a 95-00 on any thief skill is a failure, regardless of the thief's skill rating.
Backstabbing is the only way a thief can match the damage-dealing capabilities of a fighter. In order to backstab, the thief must be behind their intended victim, and attack with a bladed weapon that can be held in one hand. In order to backstab, the enemy must be unaware of your presence and your impending attack. They do not have to be completely unaware of any danger, but they must not know that you are behind them and are about to stab them. When backstabbing, the enemy's bonuses to AC due to dexterity and shield are negated, as with all rear attacks. The thief also gets a +4 bonus to-hit rather than the normal +2 bonus to-hit for a rear attack. If they land a blow, they multiply their damage by their backstab multiplier.
Backstabbing can be done with any bladed weapon that can be wielded in one hand. The size of the weapon affects how stealthy the backstabbing can be: a longsword (or similarly sized weapon) will definitely reveal the thief to anyone nearby if it is used to backstab - it is hard to quietly run someone through. A short blade will require an additional Move Silently check from the thief to kill with covertly. Daggers and knives are the best for stealthy killing, and require no additional checks.
Passing the pick pockets roll indicates that you have successfully stolen the object, but failing does not necessarily mean that you have been caught. A failed roll entitles the victim to an intelligence check; if they pass, they detect your attempt. For every 2 levels you have above your victim, they receive a -1 penalty to this check.
For a non-thief picking pockets, an opposed dexterity vs. intelligence check is called for. Regardless of level, no penalty to the intelligence check is imposed, and the non-thief receives a -2 penalty to this check unless they have the Sleight-of-Hand NWP (or similar as adjudicated by the DM). Losing the contest means that the non-thief is detected every time.
Picking a lock takes 1d10 rounds, for which an open locks check is made. If passed, the lock is opened. If failed, a new attempt cannot be made. A roll of 95-00 breaks the picks being used. Improvised picks can impose a -5% to -60% penalty. A poor lock gives a +10% bonus to open locks, while a good lock imposes a -20% penalty. The best locks can impose as much as a 40% penalty to the check.
Searching for a trap on an object or area takes 1d10 rounds; alternatively, a thief can check their immediate surroundings for traps while on the move if they move at half-speed. The DM rolls the check so that the player is not aware of whether or not they passed or failed. Magical or invisible traps are checked at 1/2 the chance. Removing a trap requires another roll and another 1d10 rounds. If failed it is usually obvious that the trap is not disarmed, and the thief may not attempt another F/RT check against it until they attain another level. A roll of a 95-00 when removing a trap indicates that the trap was accidentally activated.
Move silently allows a thief to move with complete silence regardless of their gear, so long as they are wearing no armour, leather, or studded leather armour (or a variant thereof). Move silently is not required to move silently through the bushes in a combat, or to simply move around in a stealthy manner. Sneaking directly behind someone to backstab them or sneaking right past a guard, however, requires a move silently check. When moving silently, the thief moves at 1/3rd of their normal movement. Moving silently in plain sight is pointless; there is no thief skill associated with not being seen (hide in shadows is not used for this purpose - see below); whether you are in the line of sight of a creature determines whether you can remain undetected with move silently, and usually trying to sneak within someone's line of sight is completely futile. For uncertain situations, it is up to the DM to decide whether you can sneak past someone. For example, if there is not much distance to cover, the DM may allow a thief to move silently past a guard whose attention is temporarily distracted.
Hide in Shadows
Hide in shadows is not used in conjunction with Move Silently in order to sneak; whether or not you can be seen while moving silently is a binary thing related to line of sight. Nor is it to do things like hide behind a large box; anyone can hide behind something bigger than them. Hide in shadows is used to hide in shadows or some other kind of concealment - bushes, a curtain, or similar. Hiding in shadows cannot be done while moving, although small movements such as drawing a weapon may be done slowly. You cannot hide in shadows while you are actively being watched, although if a creature watching you is distracted (by battle or something else) you may find a hiding place and make a check to hide there. Hiding in shadows is not required in total darkness, for there is no shadow to hide in - it is simply unnecessary. Those who are hidden in the shadows are not invisible to infravision, but this makes little difference as you cannot hide in shadows in complete darkness - hiding in shadows usually necessitates there being enough light to cast shadows, and thus ruin infravision anyway.
The best way to think of it is this: do they have a reason to be looking in your direction? If so, you had better hide in shadows or they will see you. If the answer is no, then they do not see you. If they do not have a reason to be looking in your direction but are close enough to hear you moving, you need to roll for move silently.
Detect noise is an active skill, not a passive one. It does not apply to normal sounds that anyone can hear, but with a successful check they can deduce the exact number of people behind a door, hear faint sounds that would normally be inaudible or even make out a conversation that would be too faint to otherwise understand. A thief needs silence in his immediate surroundings to detect noise.
A thief's ability to climb walls is above and beyond that of a normal person. With a successful check, they can climb surface too smoth for a normal person to ascend, and can do so without climbing gear or equipment. The results of failing a check can vary; on a rough surface that a normal person would have a chance at climbing, he may be allowed a dexterity check or another climb walls check to catch himself, but on a smooth surface that a normal person would be incapable of scaling without special gear, he may well fall. Depending on the surface he is climbing, there are various modifiers to his Climb Walls check. Each of the modifiers given below can act to improve or worsen the chances of climbing a wall.
|Rope and Wall||+55%|
|Abundant Handholds (brush, tree, ledges)||+40%|
|Climber Wounded to 1/2 HP||-10%|
|Slightly Slippery (wet or crumbling)||-25%|
|Slippery (icy or slimy)||-40%|
Only a single read languages roll can be made for a particular document; if failed, the thief must wait until they attain another level of experience to try again. The amount of information they puzzle out is roughly equal to their percentage chance; with a 20% chance, they understand roughly 20% of the information.
Thief skills allow thieves to perform feats which are above and beyond what a normal person is capable of; they are honed through training and practice. A thief who is Moving Silently is not the same as a warrior trying to sneak up on someone, and even a thief who fails their MS roll might still be able to catch someone unawares if they're lucky.
Even without the Move Silently skill, its possible for a character to attempt to sneak up on or past someone. A character without Move Silently is never going to be able to noiselessly pad behind someone and put a knife in their back, of course. The limits of sneaking for a non-thief include feats such as getting the drop on an enemy, following them quietly at a distance, or creeping past them at a distance of 10 feet or more.
For non-thieves, sneaking is adjudicated with a surprise roll. If they're attacking, surprise indicates a free round of attacks as usual. Otherwise, it indicates they have successfully evaded detection - following them without being seen, getting past them, and so on. This gives non-thieves a base 30% chance of success for most humanoid creatures, but bear in mind that this can be modified by various factors such as camouflage, poor light, and fog.
It should also be noted that the base 30% chance might be given additional modifiers by the DM based on context. For example, armor does not usually affect surprise rolls - but if you're trying to sneak past someone 10 feet away, wearing noisy armor is absolutely going to reduce your chances of success.
The ability to Hide in Shadows allows a thief to hide in the darkened part of a hallway, behind a thin curtain, and in other places where a normal person would be immediately detected. As noted in the skill's description, it doesn't apply to hiding in complete darkness, or under a bed, or inside a crate. If you can completely conceal yourself from sight, then you don't need to worry about making a check.
If you are trying to hide yourself in a way that overlaps with what Hide in Shadows covers, a surprise roll is once again called for. Just as with sneaking, however, there are limits to how far a surprise roll can take you. A thief can conceal themselves in the shadow of a building and stand motionlessly while someone walks within a foot of them, but the best a warrior can hope for is for them to get within striking range before they're seen. As with sneaking, various modifiers might be applied by the DM - are you holding a shiny steel dagger while hiding in the dark? Then good luck staying hidden for long! If the situation is completely untenable, the DM might rule that it's impossible for a non-thief and that you're spotted immediately.
Thieves have the luxury of the Climb Walls skill, which lets them scale sheer walls with relative ease, even without a rope. Abundant handholds, rope and other gadgets are luxuries that make their job easier but are by no means required. Rules already exist in the PHB for climbing as a non-thief, and can be found under Chapter 14: Time and Movement. In brief, an unskilled climber has a base 40% chance of successfully climbing a wall.
For a non-thief, listening is pretty binary. Either you can hear something or you can't. Taking off a helmet or putting a cone to a door might makes previously inaudible sounds audible. Likewise, if you state that you're going to stop and carefully listen the DM might allow that some sound too faint to be audible while you were moving around can now be heard. Besides this, however, you can hear whatever the DM says you can.
Non-thieves may attempt to pick pockets using the base scores given in the PHB: 15% plus any bonuses from Dexterity. Doing so is a far riskier endeavor for a non-thief, however. A non-thief who fails a PP check does not get to make an opposed ability score check against their victim; they are automatically detected.
The Pick Lock and Find/Remove Traps skills are entirely outside of the non-thief's purview. It is not possible for an ordinary person to muddle through picking a lock or examining a complex mechanism for flaws, even with the proper equipment. It should be noted, however, that non-thieves are fully capable of identifying or removing a trap; they simply can't do so with a skill check.
For example, if you come across a skeleton with a shattered skull in the middle of a tunnel and no obvious cause of death, it would be wise for a non-thief to examine the ground and walls closely. In doing so, they may see that the floor is slightly raised. Examining the walls, they may see that there is a small hole. Even without the F/RT skill, you are quite welcome to deduce that blocking up that arrow-hole with something may protect you from harm.