Definition If A, B, C and D are points of D_{0} with no three on an hline, then the hquadrilateral ABCD consists of the hsegments AB, BC, CD and DA.
The CabriJava window on the right allows you to experiment. The three types are illustrated below.


The first type is called convex.
It is characterized by the fact that the hsegments AC and BD (the diagonals) intersect. Note that hyperbolic transformations preserve incidence, so that convex hquadrilaterals map to convex hquadrilaterals.
For a convex hquadrilateral, either diagonal divides the
The hquadrilateral Theorem 



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