1. Pairs of Angles
1.1. Complementary Angles
1.1.1. Two angles whose sum is 90° (that is, one right angle) are called complementary angles and one is called the complement of the other.
1.2. Supplementary Angles
1.2.1. Two angles whose sum is 180° (that is, one straight angle) are called supplementary angles and one is called the supplement of the other.
1.3. Adjacent Angles
1.3.1. Two non – overlapping angles are said to be adjacent angles if they have a common vertex, a common arm and other two arms lying on opposite side of this common arm, so that their interiors do not overlap.
1.4. Vertically Opposite Angles
1.4.1. Two angles formed by two intersecting lines having no common arm are called vertically opposite angles.
1.5. Linear Pair
1.5.1. Two adjacent angles are said to form a linear pair if their sum is 180°.
2. Angles made by a Transversal
2.1. Alternate Interior Angles
2.1.1. If two parallel lines are cut by a transversal, the alternate interior angles are congruent.
2.2. Alternate Exterior Angles
2.2.1. If two parallel lines are cut by a transversal, the alternate exterior angles are congruent.
2.3. Interior Angles on the Same Side of the Transversal
2.3.1. If two parallel lines are cut by a transversal, the interior angles on the same side of the transversal are supplementary.
2.4. Corresponding Angles
2.4.1. If two parallel lines are cut by a transversal, the corresponding angles are congruent.
2.5. Vertical Angles
2.5.1. Vertical angles are congruent.
2.6. Linear Pair
2.6.1. If two angles form a linear pair, they are supplementary.