Cantor's isomorphism theorem
In order theory and model theory, branches of mathematics, Cantor's isomorphism theorem states that every two nonempty countable dense unbounded linear orders are order-isomorphic. The theorem is named after Georg Cantor, who first published it in 1895, using it to characterize the (uncountable) ordering on the real numbers.
Source: Wikipedia — Cantor's isomorphism theorem (CC BY-SA 4.0)