A flat sheet of squared paper has an easy address system: go along, then go up. A ball has no edges and no corners, so it needs a different plan. The plan people settled on uses the one thing the Earth already has: a spin axis. The two places where the axis comes out are the North Pole and the South Pole.

Halfway between the poles, all the way round, runs the Equator. It is the natural starting line for one of our two numbers.