The first application of this will be thief skills. The target die will always be a d6. The skill die begins life as a d3, and progresses from there according to the following:
d3 -> d4 -> d6 -> d8 -> d10 -> d12 -> d14 -> d16 -> d20 -> d24 -> d30
Success = skill die comes up equal or greater than the number showing on the target die
Failure = skill die comes up less than the number showing on the target die
Super-Fail = skill die does not exceed half the number on the target die
In the tradition of JimFirePrincess, all characters may attempt to use the thief skills, testing a d3 against the target number rolled on the d6, but only the thief may improve and use better dice. The thief gets 4 points at each level (including first), and each point may be spent to increase the die for a particular skill by one step in the dice progression listed above.
Here are the skills, which are more or less standard fare...
Acrobatics: Used for crazy stuff like walking a tightrope. Also used to move past enemies or escape from them without allowing them a parting shot.
Stealth: Move your speed while undetected by your enemies. This is used to move into position for a backstab.
Backstab: This skill is not rolled against the target die. Whatever die type you have for backstab is added to your normal damage die whenever you make a successful attack from behind against an enemy that is unaware of your presence. Thieves attack at +4, while other characters only get the normal +2 for attacking from behind.
Sleight of Hand: Used for picking pockets, cheating at card games, and so forth. If your test die doesn't get at least half the number on the target die, your attempt is noticed.
Locks & Traps: Used for picking locks, disarming mechanical traps, sabotaging equipment, etc. Requires thief tools.
Poison Use: Die type is rolled and added to the d20 roll when making a saving throw vs. poison while attempting to extract poisons/venoms from defeated monsters. Requires thief tools to get the extra die.
Spider Climb: Allows you to move your speed up or down sheer surfaces. Requires leather or lighter armor. Works great with your favorite falling rules!
Scroll Use: Allows the casting of magic-user scrolls, provided an actual magic-user of equal level would be able to memorize a spell of that level. Failure generally results in horrible mishaps.
For your convenience, I have listed the percentage odds of success for each of the die types against a random target number rolled on a d6 in order to help you determine if this system is one you might want to use, or what adjustments you might like to make. However, these percentages shouldn't be shared with players, as they run a high risk of having people game the system. If a player wants to do that, fine, but at least make them do the math themselves!
Skill level 0 [d3]: 33.3%
Skill level 1 [d4]: 41.7%
Skill level 2 [d6]: 58.3%
Skill level 3 [d8]: 68.8%
Skill level 4 [d10]: 75%
Skill level 5 [d12]: 79.2%
Skill level 6 [d14]: 82.1%
Skill level 7 [d16]: 84.4%
Skill level 8 [d20]: 87.5%
Skill level 9 [d24]: 89.6%
Skill level 10 [d30]: 91.7%
- This is totally untested, but I hope to present it to the party thief next time we get together and have him try it out.
- I use the cleric tables from Men & Magic for thief experience/level progression, hit dice, saving throws, and fighting capability. If you use the tables from Greyhawk or Basic D&D, the thief will gain levels (and therefore skill points) at a slightly faster rate. Probably negligible, but there you have it.
- Other stuff I considered but didn't include either for simplicity's sake, or to keep the scope of this post narrow: starting thieves with a d4 in each skill, while all other PCs begin (and remain) at d3; upping the skill points to 4 per level; having abilities affect skills - for example, a DEX of 15+ could allow some skills to start one die higher; item-based skill improvements - e.g. masterwork thief tools could give a +1 or increase the die for Locks & Traps, etc.
EDIT: changed to give thieves 4 skill points per level, added acrobatics, and added rule that skill rank cannot exceed twice the thief's level.