2. several related but distinct concepts in geometry. The ideal of the gnomon is unitary and absolute. It is the first brushstroke, the first chiselled line which records man’s entry to the world of abstracts. This clarity is so compelling to the mathematical mind that it has been invoked by different thinkers to different ends. Oenopides used "gnomon-wise" to mean simply "perpendicular," but Euclid extended this to denote a figure created by removing a small parallelogram from one corner of another to produce what might be called by an intellect less refined than his own an arrowhead or—in the Roman alphabet—an "L." Hero of Alexandria, not content with this pedestrian business of subtraction, conceived of a usage which would deal with addition: Hero’s gnomon is any thing which, when combined with another thing, yields a product similar in nature to the original entity.