## 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.