Slope of a straight line

The slope m tells how steep a line is. Take two points (x₁, y₁) and (x₂, y₂) on it. The rise is y₂ − y₁, the run is x₂ − x₁, and m = (y₂ − y₁) ÷ (x₂ − x₁). It does not matter which point comes first, as long as you use the same order on top and bottom. The same slope is also m = tan θ, where θ is the inclination angle, measured from the positive x-axis up to the line. A line rising to the right has m > 0 and θ < 90°; a line falling to the right has m < 0 and θ is between 90° and 180°, found as 180° + tan⁻¹ m (when the calculator gives a negative angle). A horizontal line has m = 0 (θ = 0°). A vertical line has no slope.