A d100—also called a d%, percentile die, or a hundred-sided die—is a die that has an equal chance of rolling the integers from 1 to 100. The typical form of the d100 is a pair of d10s, one of which represents the "tens" digit and the other representing the "ones" digit.
Form from the standard setEdit
The d100 from the standard 7-dice set is a pair of 10-sided dice (in the standard pentagonal trapezohedral shape), one of which is numbered with the integers from 1 to 10 and the other of which has faces numbered in multiples of 10 (00, 10, 20, 30 and so on to 90). When rolled and added together, this gives an equal chance of every number from 0 to 99. A total result of 00 is usually read as 100, although that does depend on the game system.
Other pairs of d10sEdit
Any pair of d10s can be used as a d100 if one is multiplied by 10 to count as the tens digit. The dice may be a different colour to indicate which is the "tens" and which the is the "ones".
Before the development of the trapezohedral d10s, the most common form of the d100 was to have two twenty-sided dice (the same icosohedron shape as the d20) that were marked 0–9 twice. This form predates Dungeons & Dragons, being used for wargaming in the 1960s, including in naval wargaming situations by the US Naval War College as far back as 1963.
Twenty-sided dice marked 0–9 were adopted in a limited capacity by hobby wargamers in the late 1960s and early 1970s. An advertisement in a 1971 issue of Wargamer's Newsletter offered a pair of 20-sided dice for £0.42 (GBP) or $1.20 (USD), sold as percentage dice. These were the dice that Gary Gygax obtained and used as d20s, first in his World War 2-era game Tractics, and later in D&D.
Some dice have been manufactured that actually have 100 sides. One such is known as the zocchihedron (after its creator, Lou Zocchi), which is not a true polyhedron but rather a sphere with a hundred flattened planes on its surface. For that reason, it is sometimes also referred to as a golfball. Zocchihedra are often prohibitively expensive and have a habit of continuing to roll until they fall off a table, so they are not in widespread use.
A few systems use d100 for their random resolution mechanics, including Eclipse Phase, Unknown Armies, Palladium's Megaversal games, RuneQuest, Rolemaster, Call of Cthulhu, Anima, and Warhammer 40,000 role-playing games like Rogue Trader and Deathwatch. Such games are more often roll-under (i.e. lower scores are better).
In other game systems in which the d100 is not the main resolution tool, d100s can still be used for other random effects, including rolling on random tables, e.g. for random encounters. Early editions of Dungeons & Dragons used a percentile system for bonuses to Strength above 18.