Here we review the definitions of certain special angles and properties of angles.
Definition: When the rays are the two halves of a line (they point in opposite directions), the angle is called a straight angle:
A straight angle measures 180o.
Definition: When the sides of an angle are perpendicular, the angle is called a right angle:
A right angle measures 90o.
Definition: If the measure of the angle is less than 90o the angle is called an acute angle:
Definition: if the measure is more than 90o the angle is called an obtuse angle:
Definition: When a point is the vertex of the angle or on a side of the angle, it is said to be on the angle.
Definition: A point is interior to an angle if it lies on some segment with endpoints on the sides of the angle. An object such as a ray or segment is interior to an angle if all its points other than its vertex or vertices are interior to that angle (a vertex can be interior or on the angle, as long as the other points are interior).
For example, point P is interior to 
because it is on segment 
, where D and E are points on the sides of the angle, and the whole segment 
is also interior:
Definition: A point, ray, or segment is exterior to an angle if it is not interior to that angle.
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Date last modified: September 9, 2008.