The Coordinate Plane

A city grid: 4th Avenue and 9th Street pinpoints one corner. A game of Battleship: B-7 fires at one square. Both use two numbers because a flat surface needs two directions to locate a point. Math formalizes this as the coordinate plane — two perpendicular number lines crossing at zero — and it's where algebra becomes geometry. Every line, parabola, and wave you'll graph lives here.