Using and classifying angles in geometry is a lot like the basics of lines  they both form an important building block for later lessons in Geometry. If you are not familiar with line terminology or want a quick refresher, visit theGeometry Vocabulary section! The angles we'll be working with here are quite simple, and there are only a few of them worthy enough of remembering!
Here they are:
1. Right Angle. This angle is usually denoted with a little box in the corner of the angle, and it is always 90 degrees.
2. Acute Angle. This angle simply has an angle less than 90 degrees. Since the angle is smaller, the two endpoints are closer together.
3. Obtuse Angle. An obtuse angle has an angle greater than 90 degrees. Because the angle is larger, the two endpoints are further apart.
In the graphic above, the red line and the bottom line form an obtuse angle. The bottom line and dashedline form a right angle, while the lower black line and the bottom black line form an acute angle.
Those are the three ways to identify an angle. There are a few other classifications, too.
1. Congruent Angles. "Congruent" means "equal," so congruent angles are angles equal to each other. (Yes, that's it!)
2. Complimentary Angles. These are two angles that add up to 90 degrees. The angles themselves may or may not have any additional relationships to each other; they just need to add up to 90 degrees. (45 and 45 degrees, 60 and 30 degrees, 10 and 80 degrees, etc.)
3. Supplementary Angles. Similar to Complimentary Angles, but supplementary angles add up to 180 degrees. Nothing else is required  only two angles that add up to 180 total degrees. (90 and 90, 70 and 110, etc.)
That's all that's to it! After this lesson we get into some more nitty gritty details of Geometry, including finding the Area of Shapes, as well as the Volume, and more! Review all the Math Topics at the Math Lesson Center or select another subject at the Lesson Center!
