Recall that an angle is made of two rays with a common vertex. The rays are the sides of the angle, and they go on forever. This means it doesn't matter how long you draw those rays, so the following two angles are really the same size even though they look different:
Angles are sometimes labeled using letters of three points on them, the vertex and one on each side of the angle. The vertex-letter is always the middle letter. There are two names we could give the following angle. It could be labeled 
or 
:
When there is no confusion, we can also use just the vertex to name an angle. In the above case, we could name the angle as 
.
The size of an angle depends on how much its sides slant away from each other and is usually measured in degrees. Degrees are determined by drawing a circle centered at the vertex of the angle, placing 360 equally-spaced marks on the circle, and counting the number of marks on the shorter arc of the circle between the sides. Such a circle is called a protractor. Since the shorter arc is always used, protractors are usually constructed as semicircles (half circles) instead of full circles, and an angle cannot contain more than 180 degrees.
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Date last modified: September 9, 2008.