Before working through the information below you may wish to review this lesson on measuring angles as well as taking a look at the lesson on adding and subtracting angles.

This lesson will provide information and guidance on:

- Complementary Angles,
- Supplementary Angles,
- Opposite Angles, and
- Corresponding and Alternate Angles.

**Parallel Lines** - lines that are equidistant from each other and never intersect.

**Transversal **- a line that intersects two or more other lines.

**Adjacent Angles** - angles that share a common side and that have a common vertex.

Complementary Angles are those which add together to make 90°.

∠ABD + ∠DBC = 90° | |

These two angles are complementary because they add together to make 90°. 60° + 30° = 90° | |

These two angles are also complementary. 15° + 75 ° = 90° |

The examples above all show two angles that are complementary. Notice that the angles do not have to be adjacent to be complementary. If they are adjacent then they form a right angle.

Supplementary Angles add together to make 180°

125° + 55° = 180° |

The two angles shown above are supplementary to each other. They add together to give 180°. They can be said supplement each other. Note that, as with complementary angles, they do not need to be adjacent to each other.

When to lines intersect they create four angles. Each angle is opposite to another and form a pair of what are called opposite angles.

Angles a and c are opposite angles. Angles b and d are opposite angles | Opposite angles are equal. The two 130° angles are opposite as are the two 50° angles. |

Opposite angles are sometimes called vertical angles or vertically opposite angles.

The example below shows two parallel lines and a transversal (a line that cross two or more other lines). This results in eight angles. Each of these angles has a corresponding angle. Looking at the two intersections, the angles that are in the same relative (or corresponding) positions are called corresponding angles.

Since the two lines are parallel, the corresponding angles are equal.

a and e are corresponding angles b and f are corresponding angles c and g are corresponding angles d and h are corresponding angles |

As Shown below, there are also two pairs of alternate interior angles and two pairs of alternate exterior angles. Notice how the interior angles are in between the two parallel lines and the exterior angles are to the outside.

a and g are alternate exterior angles b and h are alternate exterior angles c and e are alternate interior angles d and f are alternate interior angles |

Since the two lines are parallel, the alternate angles shown above are equal.