In the plane, if a line intersects one side of a triangle and misses the three vertices,
then it must intersect one of the other two sides.
This is a special case of the generalized Menelaus'
theorem with 
.
Pasch's Axiom
See also
Helly's Theorem, Menelaus' Theorem, Pasch's TheoremExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Pasch's Axiom." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/PaschsAxiom.html