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.   
< ----- 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