Two arrows with different arrowheads make identical lines look different lengths

In the Müller-Lyer illusion, a line capped with outward-pointing fins looks longer than an identical line capped with inward-pointing fins. It's one of psychology's oldest visual illusions, and later cross-cultural studies found it's far weaker among people who grew up without rectangular, 'carpentered' architecture around them.

— Franz Carl Müller-Lyer, Optische Urteilstäuschungen — Archiv für Physiologie, 1889