Square

The sqaure, or regular tetragon is the 4-sided regular polygon. It has 4 equal sides and 4 equal angles.

Vertex coordinates
The vertices of a square of edge length 1 centered at the origin are (±1/2, ±1/2).

Other kinds of quadrilaterals
Besides the regular square, there are various other kinds of quadrilaterls:

Rectangle
A rectangle has two pairs of opposite parallel sides, with all angles still being equal at 90º.

Bowtie
A bowtie, also known as a crossed rectangle, is a type of crossed quadrilateral where the vertices form a rectangle. It has one pair of opposite parallel sides.

Trapezoid
A trapezoid is a quadrilateral with one pair of parallel sides. The two vertices on either end of one of the parallel sides are identical.

Butterfly
A butterfly, also known as an antiparallelogram, is a type of crossed quadrilateral where the vertices form a trapezoid. Angles and edges come in two types.

Crossed Trapezoid
A crossed trapezoid is a type of crossed quadrilateral where the vertices form a trapezoid. Angles come in two types, but unlike the butterfly, edges come in three types.