Classifying two-dimensional shapes

Two dimensional shapes

The first classification of two-dimensional shapes we should note separates out those with only straight edges from those, such as circles, semicircles and ellipses, which have curved edges.

A two-dimensional closed shape made up entirely of straight edges is called a polygon. The straight edges are called sides. It’s probably safest to restrict the use of the word ‘side’ to the straight edges of a polygon. It would not be appropriate, for example, to refer to the circumference of a circles as a ‘side’, so I’m a bit perplexed when a circle is referred to in some texts as a shape ‘with one side’.

Polygons can then be further classified depending on the number of sides: triangles with three sides, quadrilaterals with four, pentagons with five, hexagons with six, heptagons with seven, octagons with eight, nonagons with nine, decagons with ten and so on.