Axiom:Outer Transitivity of Betweenness

Axiom
Let $a,b,c,d$ be points.

Let $\mathsf{B}$ be the relation of betweenness.

Let $\ne$ be the relation of being distinct.

This axiom asserts that:


 * $\forall a,b,c,d: \mathsf{B}abc \land \mathsf{B}bcd \land b \ne c \implies \mathsf{B}abd$

Intuition


Let $abc$ and $bcd$ be line segments.

Suppose they are arranged to form one straight line segment.

Then the points $a,b,d$ are collinear.

Note that this axiom still holds in the degenerate cases where not all the points are not distinct.

For example, if we are dealing with exactly three points, this axiom could be interpreted as "three points on a line are collinear".

Also see

 * Inner Transitivity of Betweenness