## Types of Angles

Angles are one of the most basic concepts in geometry, and there are many geometry terms that refer to them. *Acute* angles are less than 90 degrees, *obtuse* angles are more than 90 degrees, and *right* angles are exactly 90 degrees. If two angles are *complementary, *they add up to 90 degrees, and if two angles are *supplementary, *they add up to 180 degrees.

## Types of Triangles

Triangles are defined by the types of angles that make them up. A *right* triangle has one right angle and two acute angles. An *isosceles *triangle has two equal angles (and two equal sides as well). An *equilateral *triangle has three equal angles (and three equal sides as well).

## Types of Lines

There are two main geometry terms referring to types of lines that are important to know: parallel and perpendicular. *Parallel* lines are lines that will never meet, no matter how far they are extended both ways (like the opposite sides of a square). *Perpendicular* lines are lines that meet at a 90 degree angle (like adjacent sides of a square).

## Types of Shapes

The study of geometry is often thought of as the study of shapes. Everyone knows the definition of *triangle*, *circle*, and *square*, but what about *polygon*, *octagon*, and *pentagon*? Just break apart the word into prefixes and roots to find that *polygon* is a shape with many sides, *octagon* is a shape with eight sides, and *pentagon* is a shape with five sides.

## Types of Measurements

Some of the most basic geometry terms have to do with measuring parts of a shape. The *perimeter* of a shape is the distance around the lines that make it up. The *area* of a shape is the measurement of the space inside of it. The *circumference* of a shape is essentially the perimeter, but it can only refer to the “perimeter” of a circle.

## Circle Terms

In addition to circumference and area, there are three other main terms that apply solely to circles. The vertex of a circle is the point in the direct center of the circle. The *radius* of a circle is the distance from the vertex to any point on the circle. The *diameter *of a circle is the distance from one point on the circle to another, passing through the vertex. The diameter of a circle cuts a circle in half, and it is always twice the length of the radius.