The story goes that, while watching a fly buzzing round the room (or crawling over the ceiling, depending on which version you prefer), the idea occurred to René Descartes that its position in space could be precisely defined by specifying just three mutually orthogonal measurements made from a fixed point. Thanks to that fly, we now have “Cartesian coordinates”.
René Descartes was really quite smart.
When he noticed a fly, René said:
“Mon Dieu! Its position
Is described with precision
By coordinates: x, y and z.”
[For American readers: the letter z is pronounced ‘zed’ in Britain.]
There may be an alternative explanation: see “Helpful Helena”.