Where in a variable number of dice are used, the number determined by some attribute or combination. Usually, only one kind of die is used.
Roll and Total
The dice in the pool are all totaled before reading, then compared to a target nuber set by difficulty. WEG's d6 system is the best known, but also several other games used similar, including Space:1889 and BTRC's EABA. Axiomatically roll-high.
Roll and Keep
A number are rolled based upon one stat, and a number totaled based upon a different stat. Best known example is L5R, but other games have used the mechanic; EABA uses a roll based upon skill, keep is best 3 in almost all cases, but a trait allows keeping a fourth in some cases. Almost every game using it is roll-high.
Roll, read best only
The dice pool is assembled, rolled, and a single die is read, and compared to a target number. A shocking number of variations on this exist. Savage Worlds is usually 2 dice rolled, 1 being a d6, the other by stat or skill; maximum counts as maximum plus another die. Chronica Feudalis is Skill and tool dice, possibly also a die from an advantage, read best.
Orcworld and Silhouette (Jovian Chronicles, Tribe 8) both have multiples count as more than the face but read as if a single die. In Silhouette, only 6's do that, so a roll of 3 6's is an 8, but a 6 and 3 5's is a 6. In Orcworld, 3 6's is an 8, but 3 5's and a 6 is a 7 (5+2).
Best Two and...
Cortex Plus (Marvel, Smallville) uses 3 or more dice, with the best two roll dice being totaled, and the largest remaining die (number of sides, not roll result) being how much effect was done.
Each die is individually checked against a target number.
Vampire: the Masquerade: in the original appearance, the difficulty set a target number, and the number of dice which exceeded that TN was the quality of the result. Shadowrun also used this model.
World of Darkness in general moved to a fixed TN - tho it varied being either 6, 7, or 8, with difficulty being the number of successes that got ignored.
Arrowflight used a roll-low variation: stat dice for skill or less each, with difficulty modifying the skill.
Mixed pool; grab skill dice by skill, stat dice for stat, difficulty dice for difficulty, etc. Roll them, and read the two-axis results. Successes are cancelled by failure results, advantages/boons are cancelled by threats/banes. You succeed or fail based upon successes, and for attacks, add the number of successes to the damage; the advantages represent side effects. It's possible to fail but have a strong pile of advantage left which results in favorable outcomes. Conversely, it's possible to succeed and have lots of thread/bane results that mean bad things happened to you even tho' you succeeded.
Additional symbols (Sigmar's Comet, Chaos Star in Warhammer; Triumph and Despair in Star Wars) indicate even stronger side effects, but also count as a success.
Base is a d12. Each level of skill is an addition d6. Roll and total for TN+ to succeed. Quality is determined by how many 6's rolled 6's, but only if you succeeded does quality matter.
Dice by Difficulty
The difficulty sets the die roll. Almost always, it's been roll and total, then try to hit under a target number. Harder tasks get more dice.
The Fantasy Trip, Traveller T4, and Traveller T5 all use d6's by difficulty.
Alternity is a d20 + another die by difficulty, for skill or less on the total. It ranges from -1d20 to +1d20, then increases by additional d20's.
Fixed Roll systems
Many systems have a set roll of a certain number of dice for almost all rolls.
Roll and add skill
Usually, a target number is set, a roll is made, a modifier from abilities is added, and the total compared.
- D20 system uses 1d20 + Stat Mod + Skill
- Interlock (Cyberpunk, Mekton) uses 1d10
- MegaTraveller uses 2d6+(asset1)+(asset2) with an asset being either a skill's level or 1/5 of an attribute's level.
- early D&D editions used this for combat rolls. 1d20, add modifiers, beat TN by target armor class and your level.
Roll skill or less
Many games use a fixed roll for a target number or less. Almost all use a target based upon a skill or ability, and adjust the target number for the difficulty.
- GURPS - 3d6 for skill or less
- Chaosium - d100 for skill or less
- FASA Star Trek RPG - d10 for skill or less on "routine actions", d100 for other actions.
- AD&D and Cyclopedia D&D Proficiencies are 1d20 for skill or less.
Generally, lower rolls are better. Variations exist. Some do crits on doubles, rather than some low fraction of adjusted skill. Warhammer FRP 1E and 2E do this (It's in a supplement for 1E).
High but under
Some use a fixed die, rolling equal to or less than a target number to succeed, but the quality of success is how high the successful die was. Pendragon is one such game. AD&D 2E's Psionics Handbook used a similar mode for Psionics.
Just roll, but spend stats to reroll
A few games you simply roll. The number of dice is fixed, but you can reroll if you fail if you have the stats. One such is the Dying Earth RPG.
Several games use other methods to replace dice.
Cards can be used as direct dice replacements in all of the above.
Sometimes they can add additional aspects besides just the normal modes.
A number of games replace dice rolls with Rock-Scissors-Paper. The mechanics of Masquerade (Live Action WoD) are very similar to those of the Dying Earth, but using RSP in-place of the die roll, and spending attributes to reroll.
Some games use point pools in lieu of dice. Want to succeed? Spend enough points on it. Some use pools figured by action, and spent on "attack" and "defense" sides; others the pools refresh less, say, per scene or per session, so you ration more carefully.
Fiasco uses the dice on the table for varios table lookups at key points in the game.
Dark Realms uses non-linear results - roll, look up on the table, find the result points. The table isn't linear; high rolls are not universally good nor bad. And, given that what a particular roll means varies by the difficulty, no particular number is good nor bad.