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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.
For most measurements one of the rays can be made to pass through 0 on the protractor. The angle measure can then be read directly on the measuring scale.
The protractor is then removed and rays are drawn from the vertex through the marks that were made.