A drafting triangle is a precision drafting tool for drawing lines at specific angles. Angles are measured in degrees. A full circle is the basis for the measurement of degrees. A circle has 360°. A right angle is formed by two lines that mark off one-quarter of a circle, or 90°. A drafting triangle always has one right angle 90°. This makes it possible to put a triangle against a T-square to draw vertical lines. A 30 60 90 triangle is used with a t-square or parallel straightedge to draw lines that are 30, 60 or 90 degrees. A 45 90 triangle is used to draw lines with a t-square or parallel straightedge that are 45 or 90 degrees. The adjustable triangle allows you to use a t-square and parallel straightedge to draw additional angles.