When I first started playing RPGs in the early 80s I was fascinated by the "funny dice" and how you weren't just limited to a regular "square" dice -- you had all these other shaped dice with multiple sides to use in your game. The polyhedral dice made RPGs seem a bit different from regular games, and to this day I still enjoy gaming dice.
Some RPG systems focus heavily on one or more of the dice though. Sometimes it's 10-sided dice (particularly in % system games), pools of regular 6-sided dice, or quite often over the last decade the 20-sided dice. I like using all of the dice as much as possible, so when I started looking at my own game system that became one of the goals - create something that lets you roll all those funny dice.
I'm also a fan of games that let you know whether you win or lose without making you do very much math. It's not that the math is hard, it's that it delays the moment of success or failure and shifts it from dice to math. In a Casino there's a cheer or a groan as soon as the dice at the craps table stop rolling. In many RPGs that moment could be after a few seconds (or more!) of adding things up. A Casino is more exciting than accountancy (for me anyway) so that was another goal of the game - reduce the math as much as possible, and eliminate post dice-roll math altogether.
Both of these goals led to the solution. Instead of rolling a dice, adding one or more numbers, and comparing it to some target you could front-load the math in the system by keeping a constant target number and varying the size of the dice you're rolling. This, as it turns out, is known as a dice-step system.
Relatively recently I learned that a few other RPGs also use dice-step system in various ways, most notably Savage Worlds and Serenity. What I've noticed about both though is that they still involve either shifting target numbers or post dice-roll math. So rather than move to those systems (which I considered!) I've continued working on the dX system.
The dX system (as opposed to the d4, d6, or d20 systems) does not have any modifiers added to the dice rolls, and all rolls have a constant target number of 4. This means that on a regular roll a 4-sided dice (d4) succeeds 25% of the time and a 6-sided dice (d6) succeeds 50% of the time. Dice with a greater number of sides are increasingly more successful.
To allow for more granularity in results and to reflect more complex situations in the game there are also opposed dice rolls. In these situations multiple dice are rolled with the highest result and/or the highest result that is equal or greater than the constant target number of 4 succeeds. A character with a d6 rated skill competing against two opponents with d4 rated skills would roll their one 6-sided dice and hope to exceed the rolls of the two opposing 4-sided dice.
Situations in the game that would affect a character's chance for a successful outcome modify the size of dice being rolled. The notation +d or -d is used to indicated the dice used should bump up or down "one step". With a normal set of poylhedral dice this means the steps are: d4, d6, d8, d10 and d12. A d8 rated skill with a +d modifier would indicate that a character rolls a 10-sided dice.
In some more complex situations a dice could be stepped up and down multiple times. This allows you to do all the calculations ahead of time and still use a simple dice roll with a target number of 4 to resolve the check. For example: A d6 rated skill with two +d modifiers and one -d modifier would use an 8-sided dice.
Since most dice sets do not include a d14, d16 or d18, the 20-sided dice is used for any steps above d12. Any results "out of range" of the desired dice-step are simply re-rolled. This is probably sufficient for most games and groups, but it's entirely possible to use a 24-sided or 30-sided dice for higher steps if you wanted.
Having a system that allows for d24 and d30 rolls makes me very happy. :)
For the Weird West campaign classes I've been posting, their skills are listed using a dX system notation:
Find/Remove Traps, Pick Locks: d4 (+1 at Levels 3,5,7,9)
This means that at 1st level the character would normally roll a d4, at 3rd level a d6, at 5th level a d8, at 7th level a d10, and at 9th level a d12. The target number to succeed at any of these checks would always be 4. These dice could be stepped up or down due to in-game situations affecting the complexity of the check. The GM could also choose to make any of these "opposed checks" in which they would not only need to match or exceed the target number of 4 - but also the number rolled on the opposing dice.
I've written an entire combat system using the dX system, but for our upcoming campaign I'm only going to use it for character skills and continue to use the D&D style system of d20 attack rolls, armor class and hit points. At least for now. :)