Calling the totality of points on a line a point-row, and the totality of lines through a point a pencil of rays, we say that the point-row and the pencil related as above are in perspective position, or that they are Perspectively related.
"An Elementary Course in Synthetic Projective Geometry"
Lehmer, Derrick Norman
If, now, two forms are Perspectively related to a third, any four harmonic elements of one must correspond to four harmonic elements in the other.
"An Elementary Course in Synthetic Projective Geometry"
Lehmer, Derrick Norman
If the two point-rows are Perspectively related, then their common point is evidently a self-corresponding point.
"An Elementary Course in Synthetic Projective Geometry"
Lehmer, Derrick Norman