

A few notes on the definition of a polygon that hopefully will help you remember:
 Flat  this means it's a plane figure or twodimensional
 Straight lines  these are called segments in geometry
 Enclosed  all the lines fit endtoend and form a figure with no openings.
The following figures are not enclosed and are not polygons:
The following figures are enclosed and are polygons:
Types of Polygons
There are a lot of types of polygons. Some you've probably heard of before like squares, triangles, and rectangles. We'll learn more about these and others. Polygons are named for the number of sides they have. Here is a list of polygon names depending on the number of sides they have, starting with three and ending with ten.
 3 sides  Triangle
 4 sides  Quadrilateral
 5 sides  Pentagon
 6 sides  Hexagon
 7 sides  Heptagon
 8 sides  Octagon
 9 sides  Nonagon
 10 sides  Decagon
Convex or Concave Polygons
A polygon is either convex or concave. It's convex if any line drawn through it intersects only two other lines. If any line drawn through the polygon can hit more than two other lines, then it's concave.
Examples:
Concave 
Convex 
In a convex polygon, every angle is less than 180 degrees. In a concave there is at least one angle greater than 180 degrees.
Simple and Complex Polygons
In a simple polygon the lines don't intersect. In a complex polygon the lines intersect.
Examples:
Complex 
Simple 
Regular Polygons
A regular polygon has lines that are all the same length and it also has all the same angles.
Examples:
Regular:
Not regular:
More Geometry Subjects
Circle
Polygons
Quadrilaterals
Triangles
Pythagorean Theorem
Perimeter
Slope
Surface Area
Volume of a Box or Cube
Volume and Surface Area of a Sphere
Volume and Surface Area of a Cylinder
Volume and Surface Area of a Cone
Angles glossary
Figures and Shapes glossary
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