A protractor is a tool which can be used for measuring angles or to construct angles with a specified angle measure.
Most protractors are made from transparent plastic. Many protractors come in the shape of a half circle, with one rounded part and one straight part. At the rounded part there is measuring scale and at the straight part there is a mark, indicating where the center of the half circle is, and a line, indicating the diameter of the half circle.
When a protractor is used for measuring an angle the center of the half circle is placed at the vertex of the angle. The difference between the measures where the rays that form the angle cross the measuring scale is the measure of the angle.
For most measurements one of the rays can be made to pass through on the protractor. The angle measure can then be read directly on the measuring scale.
A protractor may be used to draw two rays that together create an angle with a specified angle measure. Typically this is done by placing the protractor's straight part so that the mark is over the intended vertex of the angle and the line is in the direction in which one of the rays is to be drawn. A small mark is then made at and another at the angle measure the angle is to have.
The protractor is then removed and rays are drawn from the vertex through the marks that were made.