Study Guide

Basic Geometry - Angles


Not to be confused with angels, angles are the pointed corners of shapes. Angles can be named three different ways.

This angle could be named any of these ways:

  • ABC (counterclockwise)
  • CBA (clockwise)
  • B (just the vertex)

Angle ABC

Look Out: when naming an angle using three letters, the vertex must always be the middle letter!

Types of Angles

Acute AngleAn angle less that 90°.

Lame joke: this angle is so small and a-cute!
Angle DEF
Obtuse AngleAn angle greater than 90°, but less than 180°.Angle GHI
Right AngleAn angle that is exactly 90°.

This is commonly denoted with a small box in the corner.
Angle KLM
Straight AngleA 180° angle.

It's - you guessed it - a straight line.
ABC line
Supplementary Angles and Linear PairsAngles that add up to 180°.

Supplementary Angles RST/TSU are supplementary angles.

If the two angles are adjacent (next to each other) and add up to 180°, they are a linear pair. 

Supplementary Angles RST/TSU are also a linear pair. 
Complementary AnglesAngles that add up to 90°.

NOQ and QOP are complementary.
Vertical AnglesWhen two lines intersect, opposite angles are called "vertical angles".
Vertical angles are congruent.

angle 1 is equal and congruent to angle 3
angle 2 is equal and congruent to angle 4

See the squiggly line over the equal sign above? This symbol means congruent. Oh, and btw, congruent angles have the same measurement.