So, I’ve been thinking again (as one does) about Thieves’ skills and bonuses in OD&D and B/X lines. I wrote about this before, in It Takes A Thief back in 2008. Nowadays, Original Edition Delta has a nice simplification of calculating them to eliminate the weirdness of the percentiles (that are almost always increments of 5% anyway) by just using the Thief’s level as a modifier on a Target 20 roll, but by design it sticks very close to the RAW chances of success. The problem, for me and my players at least, is those numbers are so low for everything except climbing that the thief shouldn’t bother trying them unless there’s nothing really riding on it or they’re almost to “name” level: you don’t hit 55% in anything except climb until level 7 in OSE(B/X) . That’s pretty much the opposite of how you want a thief to play. There is a school of thought that you should just drop the Thief as a class, and “if you want to be a Thief, steal something” but I’ve encountered a lot of players over the years whose favorite class is Thief, so there’s something about the archetype that speaks to them and I want to accommodate that.
Using the interpretation that Thief skills are near-magical abilities (I think due to Philotomy: anyone can hide, but a Thief can hide in shadows) doesn’t really help. Even if you don’t mind the flavor of thieves with semi-mystical abilities, they’re still not going to do it with any degree of reliability until near the end of the campaign. Other adjustments such as allowing repeated tries or treating a miss as indicating success but after a delay, proportionate to how much you missed by kind of works for some things like picking a lock that it might be possible to attempt until you get it right… but for something like moving silently that’s no help at all, and still leaves it that you shouldn’t bother rolling unless it’s either desperate enough that you have nothing else useful you could try on your turn or you have no real pressure and the Referee should just give it to you and move on.
Taking a step back, how reliable should these skill be at first level? Well, how powerful are they? The answer is “not very.” A first level Fighter will hit an unarmored opponent about half the time (give or take about 5% depending on edition), and on the average that hit will kill a 1 HD foe. A first level Magic User can cast a spell (admittedly once a day) that can also on the average slay a 1 HD creature regardless of AC (Magic Missile) or put up to 2 dice worth of 1 HD creatures to sleep (or proportionately less up to a max of 4 HD creatures). And the first level Thief can… open a lock? Hide?
Looking at it this way, is there any reason in the game not to make the default success rate for Thieves’ abilities pretty much the same as for Fighters? About 50% of the time it works, at least in the typical situation you’d find on the first level of the dungeon or back in town? If you can actually open the chest, or find the trap, or move silently past the guard that doesn’t seem like it would break the adventure… unless your adventure assumes that the players would never be able to do that. And if you’re writing adventures like that, well I won’t tell you to stop, but maybe you should get rid of Thieves as a class.
So, where does that leave us? I think for OED, I’d just make it Target 10; for more or less by-the-book B/X or Old School Essentials you could keep the percentiles and just add 50%, but my inclination is to make it about half-way between OSE and OED. Use a d20 and resolve, but adjust the percentiles by dividing by 10% (rounding down) giving a bonus range from +0 to +10 and allowing the Thief’s Dexterity modifier to apply. The Target Number would 10 for 1st level challenges (the kind you’d find while fighting 1 HD creatures) but scale with the level of the dungeon (5th level dungeon has TN 15 locks) or the equivalent in overland/city adventures. A wealthy merchant can probably afford Target 15 locks, a 10th level Lord Target 20. That way the challenges for Thieves roughly keep pace with the challenges that the Fighters and Magic Users are facing in terms of AC and spells saves. I like that this makes it really easy to convey a sense of some tasks are harder than others even for master Thieves while still letting the players have a good guess of how hard it’s likely to be instead of springing modifiers on them lock-by-lock. Keep the usual 1 is automatic miss, 20 is automatic success from the combat system. For Pick Pockets instead of looking for rolls of more than twice the chance of success to see if the Thief is caught, I’d change it to getting caught if the Thief misses by more than the Thief’s own level.
There you have it, a pretty minimalist change that I think opens up the play possibilities for Thieves a great deal. Even a first level Thief has at least a coin-flip’s chance of accomplishing any of their core abilities, while retaining the flavor of the old school Thief with the slightly different advancement of the distinctive Thief skills.
Update: Just to make it clear, I do let non-Thieves try any of the Thief skills (except read scrolls). They roll as 0-level Thieves: 1d20 versus whatever the target is with no bonus, not even attribute modifiers. High attributes only help if you have the slightest idea how to apply them properly; I don’t want high Dex, say, to automatically be as good as a Thief who had to work for those levels.