What is a Quadrilateral?
In EQF (Encyclopedia of Quadri-Figures) a Quadrilateral is defined as a system consisting of four lines occurring in a plane, no three of which are concurrent. 
There are no points involved. There is no order in these lines. Just four random lines. Nothing more and nothing less. 
Every line in a Quadrilateral is exchangeable with one of the other lines. 
Whatever is valid for a subset of these four lines is also valid for another subset of equal amount of these lines.  
A Quadrilateral is a flexible framework that can be used to construct many objects upon. 
In EQF these objects often will be prefixed with “QL-”. 
If the four lines making up a Quadrilateral are intersected pairwise in six distinct points, a figure known as a Complete Quadrilateral results.  
A Complete Quadrilateral is therefore a set of four lines, no three of which are concurrent, and their six points of intersection.  
Each point being the intersection of 2 lines has its opposite point by intersecting the other 2 lines.  
Therefore there are 3 pairs of opposite intersection points in a Complete Quadrilateral. 
The line connecting a pair of opposite intersection points is called a Diagonal. 
A Complete Quadrilateral has three Diagonals forming the Diagonal Triangle of a Quadrilateral. 
< ----- In the column at the left several items and objects can be chosen from Quadrilaterals. 
            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