Szpilrajn extension theorem
In order theory, the Szpilrajn extension theorem (also called the order-extension principle), proved by Edward Szpilrajn in 1930, states that every partial order is contained in a total order. Intuitively, the theorem says that any method of comparing elements that leaves some pairs incomparable can be extended in such a way that every pair becomes comparable.
Source: Wikipedia — Szpilrajn extension theorem (CC BY-SA 4.0)