Modifiers are bonus (positive) or penalties (negative) that affect a die roll.
The most common modifiers are your ability modifiers, since they determine your overall strengths and weaknesses. Usually, each die roll can only benefit from a single ability modifier at a time, but other modifiers may apply.
In most cases, modifiers to a given roll stack (combine for a cumulative effect). The GM may rule that some of them are incompatible or arise from similar sources, and therefore don’t stack. In that case, only the best bonus and/or worst penalty apply.
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