Choose the probability shape, not just a maximum number
A single d6 gives six equally likely faces. Two d6 create totals from 2 through 12, but 7 has six combinations while 2 has one. A d12 reaches the same upper value as two d6 yet keeps every individual face at one-twelfth probability. Pick the page whose sample space matches the rule before a result is generated.
Totals, pools, and kept rolls answer different questions
A custom sum combines independent dice into one number. A pool keeps every face visible so a rule can count successes or inspect the highest value. Advantage and disadvantage use two d20s but retain one extreme, changing the chance of a threshold rather than adding a second value. Those are distinct operations even when they begin with similar-looking dice.
Check a rule before clicking
Write down the target, modifier, success threshold, zero convention, and treatment of rerolls before the dice appear. For example, target 15 on an unmodified d20 succeeds on six faces, or 30%; a +3 modifier changes the successful raw faces to 12 through 20. The displayed die supplies chance, while the game or activity supplies the meaning of a result.
Keep fairness attached to the candidate set
Equal face probabilities do not make unequal consequences fair. A die can allocate one of six comparable chores, but it cannot decide whether six different prizes, penalties, or safety choices should have equal weight. Define eligibility and any exception rule in advance, then preserve the result together with the rule used to read it.
Use a new roll for a new event
Independent rolls do not remember earlier outcomes. A streak can be surprising without changing the next face's odds. Repeating a roll because an outcome is inconvenient is not statistical correction; it is a new procedure. For games and group decisions, say in advance when a reroll is permitted and what happens to the first result.