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.

Trapeoid
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.