**Angle:** An angle is a figure formed by two rays with a common end point. The common end point is called the vertex of the angle and the rays are called sides of the angle

**Acute angle:** An acute angle is an angle whose measure is greater than 0^{0}, but less than 90^{0}.

**Right angle:** A right angle is an angle whose measure is 90^{0}. If two perpendicular lines intersect each other, at the point of intersection form right angles.

**Obtuse angle:** An obtuse angle is an angle whose measure is greater than 90^{0}, but less than 180^{0}.

**Straight angle:** A straight is a angle whose measure is exactly 180^{0}. If two opposite rays intersect each other, at the point of intersection form straight angles.

**Reflex angle:** A reflex angle is an angle whose measure is greater than 180^{0}, but less than 360^{0}.

**Complementary angle:** Two angles are called complementary angles if the sum of their measure is 90^{0}.

## If two angle angles are complementary then every angle is complementary to each other.

**Supplementary angle:** Two angles are called supplementary angles if the sum of their measure is 180^{0}. If two angle angles are supplementary then every angle is supplementary to each other.

**Vertical angles:** Non adjacent angles formed by two intersecting lines are called vertical angles. Two vertical angles are always congruent. In the figure ∠A and ∠B are a pair of vertical angles as are ∠C and ∠D.

**Adjacent angles:** Angles that are placed side by side are called adjacent angles. Two angles are called adjacent angles, if, and only if, they have a common vertex and a common side lying between them.

**Alternate interior angles**: If two lines in the same are cut by a transversal, the non adjacent angles on opposite sides of the transversal and on the interior of the two lines are called alternate angles. In the figure ∠3 and ∠6 are a pair of alternate angles as are ∠4 and ∠5

**Corresponding angles:** If two lines in the same are cut by a transversal, the angles on the same side of the transversal and in corresponding positions with respect to the two lines are called corresponding angles. In the figure ∠1 and ∠5, ∠2 and ∠6, ∠3 and ∠7 and ∠4 and ∠8 are pair of corresponding angles.