Editing Projective line over a ring (section)

=== Point-parallelism ===
Two points of P<sup>1</sup>(''A'') are '''parallel''' if there is ''no'' chain connecting them. The convention has been adopted that points are parallel to themselves. This relation is [[invariant (mathematics)|invariant]] under the action of a homography on the projective line. Given three pair-wise non-parallel points, there is a unique chain that connects the three.<ref>[[Walter Benz]], Hans-Joachim Samaga, & Helmut Scheaffer (1981) "Cross Ratios and a Unifying Treatment of von Staudt's Notion of Reeller Zug", pp. 127–150 in ''Geometry – von Staudt's Point of View'', Peter Plaumann & Karl Strambach editors, Proceedings of NATO Advanced Study Institute, Bad Windsheim, July/August 1980, [[D. Reidel]], {{isbn|90-277-1283-2}}, {{MathSciNet|id=0621313}}</ref>