A plastic or metallic semi-circular disc, which contains a scale marked from $0^\circ$ to $180^\circ$ and used to measure angles is called a protractor.
Protractor is a basic angle measuring tool and it is really useful to measure angles in all the cases. So, everyone must know the parts of the protractor clearly and their purposes to use it in angular measuring easily.
A protractor contains three factors and they play vital role in measuring the angles geometrically.
Two lines are intersect perpendicularly at a point in any protractor and it is called origin of the protractor.
This point is actually at middle of the protractor exactly. Hence, the origin of the protractor is usually called as centre (or) midpoint of the protractor.
A line is displayed in horizontal position, it means the line is displayed as a parallel line to the surface of the earth. On the basis of this line, the every angle is measured geometrically. Hence, the line is called as base line of the protractor.
The centre of the protractor divides this base line in two equal parts.
The baseline which is appeared left side of the centre of the protractor is called left side baseline of the protractor.
The baseline which is appeared right side to the centre of the protractor is called right side baseline of the protractor.
There are two types of scales in degrees, arranged in semi-circular structure. It displays a scale from $0^\circ$ to $180^\circ$ but one degrees scale is in clockwise direction and the other scale is in anticlockwise direction.
A semi-circular degrees scale is displayed from $0^\circ$ to $180^\circ$ and it is in anticlockwise direction. Hence, this scale is called as anticlockwise degrees scale of the protractor.
Anticlockwise degrees scale is displayed internally. Due to this reason, it is also called as inner scale of the protractor.
The right side baseline is considered as baseline while measuring an angle by using anticlockwise degrees scale.
Another semi-circular degrees scale is displayed from $0^\circ$ to $180^\circ$ but it is in clockwise direction. Hence, this scale is called as clockwise degrees scale of the protractor.
Clockwise degrees scale is displayed outer in the protractor. Hence, it is also called as outer scale of the protractor.
Left side baseline is used as baseline while measuring any angle by using clockwise degrees scale.