Angles at a Glance

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

NameDescriptionExample
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. 
RSU
Complementary AnglesAngles that add up to 90°.

NOQ and QOP are complementary.
NOP
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
1234

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

Example 1

Are the following angles complementary or supplementary?


Example 2

Are the following angles complementary of supplementary?


Example 3

Name the following angles and angle pairs in the image below:

  • an acute angle
  • an obtuse angle
  • a right angle
  • two congruent angles
  • two straight angles
  • two complementary angles
  • two supplementary angles


Refer to the following diagram to answer questions 1-4

Plane GFI

Exercise 1

Name two sets of complementary angles.


Exercise 2

Name two sets of vertical angles.


Exercise 3

Name four obtuse angles.


Exercise 4

Name four right angles.