Full Idea
There is a familiar comparison between Euclid (unique parallel) and 'spherical' geometry (no parallel) and 'saddle' geometry (several parallels).
Gist of Idea
Euclid has a unique parallel, spherical geometry has none, and saddle geometry has several
Source
William D. Hart (The Evolution of Logic [2010], 2)
Book Reference
Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.44