Introducing variability into a TTRPG is a key part of what makes it a game rather than just a series of agreed upon rules and guidelines. Most often, this is done via dice, but it can also be done via a deck of playing cards, a deck of Tarot cards, or other means.
Dice
Dice have a long history of being used, and the variety of mechanics out there make it hard to list all of them. The notation used for dice is ndx, where n is the number of dice, and x is the number of sides the dice has. 1d20 would be one twenty-sided die.
In addition, consider the concept of Difficulty Class, or DC for short. This concept has many different names across different games, but DC is the term being used for this particular discussion.
WARNING! NUMBERS BELOW!
1d20 Roll Over
The DC in this instance is usually arbitrary, but the goal is to roll at or above the given number. Occasionally, this is further enhance with 1 being the worst possible result and 20 being the best possible result.
There is a 5% chance to roll any of the values on a dice. The checks in Dungeons and Dragons 5th Edition, for example, range from 5 (very easy) to 30 (nearly impossible) in increments of five. This means that, assuming a game statistic has a value of 0 or no influence on a check, there is an 80% chance to succeed a very easy check. On the other hand, there is a 0% chance to succeed a nearly impossible check without any further influence.
Another concept often used in 1d20 Roll Over is the advantage/disadvantage system, where you roll twice and take either the higher number (advantage) or the lower number (disadvantage). In the above example, there is a 96% chance of succeeding a very easy check, but still a 0% chance of succeeding a nearly impossible check.
1d20 Roll Under
The DC for for 1d20 Roll Under is usually not arbitrary or left wholly up to the whims of a GM. This is true in general of Roll Under systems. For example, the system I am currently working on (Yet Another Simulation System) uses this and has all checks with a base DC of 10 modified primarily by the character’s own abilities. There are also stages of success that mark how well or how poorly you do and it scales by how you roll versus the DC.
With YASS as an example, if you attempt an unmodified check (DC 10), you’d have to roll at or under a 10. That puts you at a 50% chance of success, assuming your GM doesn’t make the check more difficult.
YASS also has something like advantage and disadvantage, except the former is “roll twice or more and take the lowest number” and the latter is “roll twice or more and take the highest number.” Assuming it’s roll twice and take lowest, there is a 75% chance to succeed; roll twice and take highest, only a 25% chance.
3d6 Roll Under
This is the mechanic GURPS uses. GURPS is a difficult beast to explain, so I encourage you to grab the GURPS 4th Edition Basic Set. In short, you want to roll at or under the effective skill.
The average DC of a GURPS check is 10. The minimum you can get is 3, and the maximum, 18, when you roll 3d6. There is a 50% chance you will succeed such a check.
1d6 vs Two 1d10
Ironsworn and its creator Shawn Tomkin brought this relatively new mechanic to the forefront. There are Move Dice (1d6) and your Challenge Die (two 1d10). The reason I specify it as two 1d10 instead of 2d10 is because you do not sum the results of the two d10s, but see how your 1d6 did in comparison to each d10. The maximum value of your Move Score is 10 – that is a perfect 6 on the Move Dice plus modifiers up to a value of 4, for a total of 10.
A strong hit (very positive) is when your Move Score is higher than both of the Challenge Dice. Without a modifier, there is about a 40% chance of success (with about 9% of that being a strong hit); with a modifier of +4, there is about an 84% chance of success (with about 45% of that being a strong hit).
nd6 vs y Successes
This one takes some explaining. All the previous examples do not use something called dice pools, where the number of dice you use increases as you become more skilled. In some systems, you need to roll a certain number of successes (y), and your opportunities to do so increase with your skills, abilities, or other game statistics (n).
In one game I am thinking of – a fan game – two skills are added together to give you the number of d6s you will roll. The GM (or Storyteller, in this case) will tell you how many successes you need based on how difficult the attempted action is, considering everything. It ranges from troublesome (1 success) to almost impossible (5+ successes). In this system, a success is a roll of 4 or higher (4, 5, 6) on a d6.
If a check has a DC of 1, and you have – combined – one skill point across two skills that is applicable, you would roll 1d6 and check for a 4, 5, 6 – rolling such would give you 1 success and you would succeed the check. There is a 50% chance this would occur. The odds, based on my limited understanding of statistics, appear to get worse the higher the number of successes required are. At 5 skill points across two skills (5d6) on a check that requires at least 5 successes, you only have about a 3% chance to succeed.
Cards
This is a whole other section that likely deserves its own post. Just know that there are several ways to handle suits, numbers, and face cards, and – in the case of Tarot cards – Major and Minor Arcana. Cards are usually used for more narrative games.
“Hey, what about…!”
There are many other kinds of random generation out there, from 1d6 to spinning bottles to flipping through books. Whatever the form, above are some of the most common dice mechanics in TTRPGs and the odds of their successes. You may want to read up on Gateway TTRPGs before you select a dice mechanic best for you, specifically the section on success.
Which dice systems do you love? Which ones do you want to try? Which do you think would be best for the game you’re working on?
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