Simple geometric shapes are studied in mathematics partly because they are used in thousands of practical applications. For instance, triangles occur in bridges, pylons and, more mundanely, in folding chairs; rectangles occur in windows, cinema screens and sheets of paper; while circles are an essential part of wheels, gears and plates.

By definition, triangles are shapes with three straight sides. However, there are various types of triangle:

An equilateral triangle is a triangle with all three sides of equal length. The three angles are also all equal.

An isosceles triangle is a triangle with two sides of equal length. The two angles opposite the equal sides are also equal to one another.

right-angled triangle is a triangle with one angle that is a right angle.

scalene triangle is a triangle with all the sides of different lengths. The angles are also all different.