The Lesson
A polygon is a 2-dimensional shape with straight sides.Dictionary Definition
The Oxford English Dictionary defines a polygon as "a plane figure with many (usually more than four) straight sides and the same number of angles."Regular Polygons
Regular polygons have sides of equal length and equal angles.Shape | Name | Number of sides |
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Triangle (or Trigon) A regular triangle is an equilateral triangle | 3 |
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Quadrilateral (or Tetragon) A regular quadrilateral is a square | 4 |
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Pentagon | 5 |
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Hexagon | 6 |
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Heptagon | 7 |
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Octagon | 8 |
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Nonagon | 9 |
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Decagon | 10 |
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Dodecagon | 12 |
Irregular Polygons
Irregular polygons have sides of different lengths and different angles.Shape | Name | Number of sides |
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Irregular Quadrilateral | 4 |
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Irregular Hexagon | 6 |
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Irregular Octagon | 8 |
What's in a Name?
Polygon comes from the Greek 'poly-', meaning "many", and '-gon', meaning "angle". Therefore 'polygon' means "many angles" as each polygon has several angles inside of it.Interior Angles
The angles between the sides in a polygon, inside the shape, are called interior angles.

Sum of interior angles = (n − 2) × 180°
Sum of interior angles = (6 − 2) × 180°
Sum of interior angles = 4 × 180°
Sum of interior angles = 720°
Exterior Angles
The angles between the sides in a polygon, outside the shape, are called exterior angles.