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.

< ----- In the column at the left several items and objects can be chosen from Quadrangles.
By clicking at a button at the top of this colomn also objects can be chosen from:
•   INDEX QA1, QA2  -> Quadrangle-objects-1 and -2
•   INDEX QL1, QL2  -> Quadrilateral-objects-1 and -2
•   INDEX QG1, QG2 -> Quadrigon-objects-1 and -2