Pappus's Harmonic Theorem

$AW$, $AB$, and $AY$ in the above figure are in a Harmonic Range.

See also Ceva's Theorem, Menelaus' Theorem

References

Coxeter, H. S. M. and Greitzer, S. L. Geometry Revisited. Washington, DC: Math. Assoc. Amer., pp. 67-68, 1967.