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A 12-sided shape is called a dodecagon. A dodecagon is a type of polygon, which is a two-dimensional shape comprised of straight lines. These lines connect to each other to form a closed shape.

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A 20-sided shape is known as an icosagon. The icosagon is a polygon, and the sum of its interior angles is equivalent to 3,240 degrees.

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A six-sided shape is known as a hexagon. This refers to an item with a flat shape that has straight sides.

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A seven-sided shape is called a heptagon. Some people refer to heptagons as septagons, but this mixes a Greek suffix with a Latin prefix, so it is not the preferred usage.

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The name of an eight-sided shape is an octagon. In addition to the eight sides, the octagon also has eight vertices and eight interior angles. The interior angles add up to 1080 degrees.

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A nine-sided shape is called a nonagon or enneagon. A nonagon is a type of polygon. Polygons are closed-plane figures that are made of line segments.

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In geometry, a shape with five sides is called a pentagon. A regular pentagon has five sides of equal length, as well as five equal angles. A standard "STOP" sign in the US is a regular pentagon, as is the Pentagon building in the District of Columbia.

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A four-sided shape is called a quadrilateral. All quadrilaterals have straight sides, are two-dimensional, have four vertices and have interior angles that add up to 360 degrees. Types of quadrilaterals include the square, rectangle, rhombus, parallelogram, trapezoid and kite.

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A shape that has three unequal sides is a scalene triangle. A shape that has four unequal sides is a quadrilateral. A shape that has more than four unequal sides is an irregular polygon.

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A shape that has six sides is called a hexagon. A hexagon is a flat polygon with straight sides.