wiki/Gnomon 'The gnomon is the part of a sundial that casts the shadow. Gnomon (γνώμων) is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals." Also, gnomon is the name given to an aesthetic process...whereby the whole of the character is revealed by a single part.'
wiki/Gnomon_(figure) 'In geometry, a gnomon is a plane figure formed by removing a similar parallelogram from a corner of a larger parallelogram. More generically, the term gnomon denotes the form that is to be added to a figure to produce a larger figure of the same shape.'
NASA.gov 'A vertical pole of known length embedded into a perfectly level plot of ground or slab of concrete. This device was used by the ancient Egyptians as a method of timekeeping.'
Brittanica.com 'The gnomons include all of the odd numbers; these can be represented by a right angle, or a carpenter’s square. Gnomons were extremely useful to the Pythagoreans. They could build up squares by adding gnomons to smaller squares and from such a figure could deduce many interrelationships: thus 12 + 3 = 22, 22 + 5 = 32, etc.;... In the speculation on odd and even numbers, the early Pythagoreans used so-called gnōmones. This procedure—which was so far Pythagorean—led later, perhaps in the Platonic Academy, to a speculation on “polygonal” numbers.