Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In geometry, an equilateral polygon is a polygon which has all sides of the same length. For instance, an equilateral triangle is a triangle of equal edge lengths. All equilateral triangles are similar to each other, and have 60 degree internal angles. An equilateral quadrilateral is a rhombus, which includes the square as a special case.An equilateral polygon which is cyclic (its vertices are on a circle) is a regular polygon. All equilateral quadrilaterals are convex, but there exist equilateral polygons with five sides (pentagons) which are concave, and similarly for every larger number of sides.