A set is countable if its elements can be put in one-to-one correspondence with a subset of the counting numbers. For example, the sets {0, 1, 7, −3}, {red, green, blue}, {…,−2, −1, 0, 1, 2, …}, {straight, curly}, and the set of all fractions, are countable. If a set is not countable, it is uncountable. The set of all real numbers is uncountable.