Affinity is defined here as a local infinitesimal invariance of the distance between binary associated parallel lines.
It was strongly but subtly emphasized as early as 1918 in Hermann Weyl’s RAUM ZEIT MATERIE that infinitesimal parallel displacements exist if and only if a necessary and sufficient condition of real affine geometry is satisfied. That is given parallel lines m and n two arbitrary transversals AD and CB intersect at a point P such that the ratio of AP over PD is always equal to the ratio of BP over PC.
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These absolute ratios were the starting points of later developments in tensor analysis by other mathematicians for the complex descriptions of contravariance, covariance, and general covariance. Furthermore, if the point P is located half the distance from either parallel lines then DAPB@DCPD, for all other locations of point P, DAPB~DCPD. The former denotes equality or congruence while the latter denotes inequality or similarity.


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