The Coordinate Plane
René Descartes invented the coordinate plane in the 1600s — that's why it's also called the Cartesian plane. Two perpendicular number lines (axes) divide the plane into four quadrants. The horizontal line is the x-axis, the vertical line is the y-axis, and they meet at the origin (0, 0).
Plotting Ordered Pairs
A point is located by its ordered pair (x, y). Start at the origin. Move x units right (positive) or left (negative), then move y units up (positive) or down (negative). The order matters — (3, 2) and (2, 3) are different points!
The four quadrants: Q1 (+, +), Q2 (−, +), Q3 (−, −), Q4 (+, −). Remember: "Quadrant I is where both x and y are positive."
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Mnemonic: "Run then Rise"
When plotting (x, y), first go along the x-axis (run), then go up or down the y-axis (rise). Alternatively: "x is across, y is up." The x-coordinate always comes first in the ordered pair.