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Euclid's Definition

In the words of Euclid:

And in any parallelogrammic area let any one whatever of the parallelograms about its diameter with the two complements be called a gnomon.

(The Elements: Book $\text{II}$: Definition $2$)


It can also be intuitively defined as a parallelogram with a similar parallelogram taken out of the corner.

Linguistic Note

The word gnomon is a Greek one, generally meaning marker or indicator (literally: a thing that enables something to be known), and specifically meaning the piece of a sundial that casts the shadow.

Some sources interpret gnomon to mean carpenter's square.

The words gnostic and agnostic come from the same root.

The word is nowadays rarely (if ever) encountered outside the works of Euclid; specifically, The Elements.