Principle of indifference
To coins :
A symmetric coin has two sides, "Heads" and "Tails". Each of the possible ("Heads" or "Tails") outcomes has a probability of 1/2.
But if we apply the Principle of Indifference, and if we have more than 2 possible outcomes, say, 3 possible outcomes - the coin could land on its edge, then the probability of a possible outcome (Heads, Tails or Edge) is 1/3.