The Grammarphobia Blog

The language of geometry

Q: Have you ever noticed that the adjective for “triangle” is “triangular” and for “rectangle is “rectangular”? Or that the adjective for “pentagon” is “pentagonal” and for “hexagon” is “hexagonal”? Or that the adjective for “square” is simply “square,” not “squarular” or “squaral”? What accounts for these differences?

A: The words “triangle” (three-angled) and “rectangle” (right-angled) are based on Latin roots, while “pentagon” (five-angled) and “hexagon” (six-angled) are based on Greek roots. The corresponding adjectival endings for “angled” are “angularis” in Latin and “gonos” in Greek. This probably explains the difference between the “angular” and “agonal” adjectives.

As for “square,” it’s been both a noun and an adjective since the 14th century, according to the Oxford English Dictionary, so no additional ending is necessary. It came into English from Old French.