1d2

1d2 refers to rolling one two-sided die, producing an integer result of 1 or 2 with equal probability. In practice, many players interpret 1d2 as a coin flip and use a coin or a digital random bit instead of a physical two-sided die. The term is part of the dice-notation convention used in tabletop role-playing games to indicate the number and type of dice to roll.

Physical dice: A true two-sided die is uncommon; commercially available dice are typically polyhedra with six or more faces. When a physical tool is needed, players often substitute a coin, or roll multiple dice of other types and map outcomes to 1 or 2. Some games also accept a d2 roll as a virtual result from dice rollers.

Probability and math: A fair 1d2 has two equally likely outcomes, 1 and 2. The expected value is (1+2)/2 = 1.5, and the variance is 0.25 (standard deviation 0.5). The notation is combinable, so 1d2+3 yields a result from 4 to 5, while 2d2 yields a sum from 2 to 4 with probabilities 1/4, 1/2, and 1/4, respectively.

See also: Dice notation, d3, d4, d6, coin flip, Bernoulli distribution.