Euclid was a Greek mathematician who lived around 300 BC. He wrote a long set of books called the Elements, in which he started from a few clear rules and then proved, step by step, how lines, angles, and shapes fit together. The ordinary geometry you meet at school — parallel lines that never meet, a triangle whose three angles add up to 180 degrees — is still called Euclidean geometry, after him. Later mathematicians found other geometries that do not follow those rules, so “Euclidean” now also marks the familiar kind as against those stranger ones. We say the name YOO-klid. It is a person’s name used as a label for a whole way of thinking about space, not a common noun you pluralise.