There are five sides to a pentagon. There are no straight angles in a regular pentagon (it has interior angles each equal to 108 degrees). An irregular pentagon can only have three right angles since a fourth would leave 180 degrees for the last angle, which is a straight line (540 degrees – 360 degrees). A pentagon is one of the simplest polygons. There are many ways to describe a pentagon. A common way is to say that it is "five-sided". Another way to look at it is that there are five vertices, or top points, and that they are all distinct. Or you could say that it has five angles, each of which is 1/5 of 360 degrees. A pentagram is a pentagon with 5 edges. They can be constructed by joining the midpoints of 2 adjacent sides of a pentagon. A heptagon is a seven-sided figure. Like the pentagon, it has five corners or vertices. But a heptagon also has two side angles that are less than 90 degrees, and two opposite angles that add up to more than 180 degrees. So a heptagon is not really a simple shape - although it is simpler than some other polygonal figures such as octagons and nonagons. An icosagon is a 20-sided figure like a pentagram. It has 10 corner angles and 10 side angles.