A point is said to lie between points and (where , , and are distinct collinear points) if . A number of Euclid's proofs depend on the idea of betweenness without explicit mentioning it.

All points on a line segment excluding the endpoints lie between the endpoints.

Let be a partially ordered set, and let . If , then is said to be between and . If in and there is no that is between and , then covers . Conversely, if covers , then no is between and