An infinite set is considered countable if they can be listed without missing any (that is, if there is a one-to-one correspondence between it and the set of natural numbers formula_2). Examples include the rational numbers, integers, and natural numbers. Such sets have a cardinality that we call formula_3 (pronounced "aleph null", "aleph naught" or "aleph zero"). Sets such as the real numbers are not countable, since given any finite or infinite list of real numbers, it's always possible to find a number that's not on that list. The real numbers have a cardinality of formula_4—the cardinality of the continuum.