Jim Loy
Jimloy.com: Saccheri
This site explains the Saccheri quadrilateral and the importance of his work in geometry.
Texas Instruments
Texas Instruments: Constructing Quadrilaterals
In this activity, students will construct different types of quadrilaterals from the quadrilateral hierarchy. This activity asks for constructions based on a minimal definition of the quadrilateral. The activity will reinforce the...