Four coplanar line segments linked end to end to create a closed figure.
A 4-sided polygon.
Try this Drag the orange dots on each vertex
to reshape the quadrilateral.
(If there is no image below, see support page.)
The quadrilateral is any 4-sided polygon.
But it is the parent of a whole family of figures, each with their own distinct names and characteristics.
See also Tetragon.
