The Cartesian Plane was invented by Rene Descartes, in the 17th centuary. He invented it when he was lying in his bed wathcing flies crawl across his roof and was looking for a way to find the flies exact position, so he imagined his roof was divided in small squares.

E.g. The coordinates of the fly below is 3,2. Because you go across before you go up. (A way to remember this is that you crawl before you walk.