180 degrees.
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A protractor can be used to measure an angle. An angle is basically part of a circle. A complete circle is 360 degrees. A right-angle is 90 degrees, half a circle is 180 degrees, and so on.
Place the center point of the protractor at the vertex of the angle and one of the lines of the angle along the zero line of the protractor. The measure of the angle will then be where on the protractor's angular scale the other line of the angle lies.
If the arc is circular, such a figure is a semicircle or half circle.
A 180-degree arc is also called a half-circle.
A straight angle is a half circle.