Angles are those which are formed by two rays with common vertex. If one of the ray is fixed and the other is rotated about the fixed vertex, then the angle between the two rays will measure from 0o to 360o. The angles can be classified according to their measure. Measuring angles is the part of practical geometry. We measure the angles to classify into the type of the angles. The angles can be measured to find the angles subtended according to the given condition.
For example, angle subtended by an object at the retina, angle of elevation of the top of a building, angle of depression of a ship from the top of a hill etc. Measurement of these angles will help us to find the top of a building, distance between the buildings, distance of the ship from the hill or shore etc. In this section let us see how we measure angles.

What are angles? Angles are those which are formed with two rays called as arms with a common vertex.
When one of the arm is fixed and when we rotate the other arm about the fixed vertex, the different types of angles are formed.The angles are named either by writing the vertex or by using the end points on the arms as shown below.

The following diagrams show the $\angle A$ or $\angle PAQ$
Angles

Classification of Angles

Angles can be classified according to their measure.
We can see the definition of each type of angle under the figure given below which describes the types of angles.
Types of Angles
Let us see the definitions of the types of angles.

1. Zero Angle: When the measure of the angle is 0 o, that is when the two arms coincide, the measure of the angle is 0o ( read as zero degree ).

2. Acute angle: When the angle measure more than 0 o and less than 90 o, then the angle is called an acute angle.Example: 20o , 1o , 899 .
3. Right angle: When the angle measure exactly 90o, then the angle is called a right angle or we call that the two arms are perpendicular to one another.

4. Obtuse angle: When the measure of the angle is more than 90o and less than 180o , then the angle is called an obtuse angle.
Example: 91o , 100o , 179o
5. Straight angle: When the angle measure exactly 180o , then the angle is called a straight angle.

6. Reflex angle: If the measure of angle is more than 1800 , and less than 360o , then the angle is called reflex angle.
Example: 181o , 200o, 300o , 359o.
7. Complete angle: An angle whose measure is 360o is called a complete angle. In a complete angle the moving arm would have completed one rotation about the fixed point and coincide with the fixed arm, like the zero angle.