Two triangles 
and 
are said to be perspective, or sometimes homologic,
from a line if the extensions of their three pairs of corresponding sides meet in
collinear points 
, 
, and 
. The line joining these points is called the perspectrix.
Two triangles are perspective from a point if their three pairs of corresponding polygon vertices are
joined by lines which meet in a point of concurrence 
. This point is called the perspector,
perspective center, homology center, or pole.
Desargues' theorem guarantees that if two triangles are perspective from a point, they are perspective from a line (called the perspectrix). Triangles in perspective are sometimes said to be homologous or copolar.
