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. 

Related to a Quadrilateral 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.