This diagram is an example of Desargues' theorem. Triangles ABC (green) and abc (yellow) are perspective both axially and centrally.

Central perspectivity is satisfied iff the three lines Aa, Bb, and Cc are concurrent, at a point called the center of perspectivity.

Axial perspectivity is satisfied iff the point of intersection of ab with AB, and that of intersection of ac with AC, and that of intersection of bc with BC, are collinear, on a line called the axis of perspectivity.

Image created by H McKenna using Inkscape.

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Derivative works of this file:  Desargues theorem ter.svg