The axiom defining the relationship of parallelism in various geometries. See Parallel straight lines; Fifth postulate.
[a1] | D. Hilbert, "Grundlagen der Geometrie" , Teubner, reprint (1968) |
[a2] | H.S.M. Coxeter, "Introduction to geometry" , Wiley (1969) pp. 185–186; 396–404 |