Points, planes & graphs

Two number lines make a map

Take one number line going left–right and another going up–down, crossing at 0. Now every spot on the flat page has an address: how far right (called $x$) and how far up (called $y$). We write the address as $(x, y)$ — always x first.

This is the coordinate plane, invented by René Descartes so that shapes could be described with numbers. Every chart you’ll ever read is one.