Percentile dice generate a number between 1 and 100. Some games may use percentile dice to generate numbers between 0 and 99. Although dice with 100 sides are available, these numbers are commonly generated using 2 ten-sided dice. Both dice are rolled and the result is read as a two digit number using one die as the ones digit and one die as the tens digit. Some percentile dice are marked to identify the "tens" die, (using 10, 20, 30 etc.) but any two 10-sided dice can be used if the 10's die is identified before rolling. This is usually done by color.
The most common use of percentile dice is in a "roll-under" system. In this type of system, a stat is a number between 1 and 100 which represents that character's unmodified chance of success. a stat of 25 suggests that a character will succeed around 25% of the time when attempting tasks using that stat. When a check is made, percentile dice are rolled and any value equal to or less than 25 is considered a success. This system is fairly common in older games and still appears in modern games, often as a subsystem.