Classifications of Angles

When the rays of an angle are perpendicular, the angle measures 90 degrees.

90 degree angles are called right angles.

One angle is smaller than another angle if it "fits inside" with both vertices at the same point. For example, the red angle is smaller than the blue angle:

Of course, the degree measure of the smaller angle will be less than the degree measure of the larger angle.

An angle that is less than 90^o is called an acute angle, and an angle greater than 90^o is called an obtuse angle.

Since any two rays with a common vertex form an angle, we could have an angle that measures as much as 180^o when the rays point in opposite directions:

 

Such an angle is called a straight angle, and it is nothing more than a line.

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