The probability distribution of a random variable X is symmetric if there is a number a such that the chance that X≥a+b is the same as the chance that X≤a−b for every value of b. A list of numbers has a symmetric distribution if there is a number a such that the fraction of numbers in the list that are greater than or equal to a+b is the same as the fraction of numbers in the list that are less than or equal to a−b, for every value of b. In either case, the histogram or the probability histogram will be symmetrical about a vertical line drawn at x=a.