What is a Quadrangle?

In EQF (Encyclopedia of Quadri-Figures) a Quadrangle is defined as a system consisting of four points occurring in a plane, no three of which are collinear.
There are no lines involved. There is no order in these points. Just four random points. Nothing more and nothing less.
Every point in a Quadrangle is exchangeable with one of the other points.
Whatever is valid for a subset of these four points is also valid for another subset of equal amount of these points. 
A Quadrangle is a flexible framework that can be used to construct many objects upon.
In EQF these objects often will be prefixed with “QA-”. 

If the four points making up a Quadrangle are joined pairwise by six distinct lines, a figure known as a Complete Quadrangle results. 
A Complete Quadrangle is therefore a set of four points, no three of which are collinear, and the six lines which join them.
These six lines often are called the sides  of a quadrangle.

Each line being the connection of 2 points has its opposite line by connecting the other 2 points. Therefore there are 3 pairs of opposite lines (sides) in a complete quadrangle. The 3 points of intersection per pair of opposite lines form the so called Diagonal Triangle of a Quadrangle.   

Related to a Quadrangle several point, lines, circles, conics, cubics, transformations and triangles do exist, which can be obtained from the pulldown menu at the left of this page.
An overview menu can be obtained by clicking at a corresponding link above or below this page.