In elliptic geometry, two lines are Clifford parallel or paratactic lines if the perpendicular distance between them is constant from point to point. The concept was first studied by William Kingdon Clifford in elliptic space and appears only in spaces of at least three dimensions. Since parallel lines have the property of equidistance, the term "parallel" was appropriated from Euclidean geometry, although the "lines" of elliptic geometry are geodesic curves and, unlike the lines of Euclidean geometry, are of finite length.
The algebra of quaternions provides a descriptive geometry of elliptic space in which Clifford parallelism is made explicit.
For an arbitrary point u in elliptic space, two Clifford parallels to this line pass through u. The right Clifford parallel is
and the left Clifford parallel is
Rotating a line about another, to which it is Clifford parallel, creates a Clifford surface.
The Clifford parallels through points on the surface all lie in the surface. A Clifford surface is thus a ruled surface since every point is on two lines, each contained in the surface.
- Georges Lemaître (1948) "Quaternions et espace elliptique", Acta Pontifical Academy of Sciences 12:57–78
- H. S. M. Coxeter English synopsis of Lemaître in Mathematical Reviews
- William Kingdon Clifford (1882) Mathematical Papers, 189–93, Macmillan & Co.
- Guido Fubini (1900) D.H. Delphenich translator Clifford Parallelism in Elliptic Spaces, Laurea thesis, Pisa.
- Roger Penrose; The Road to Reality, Vintage, 2005, pp.334-6. (First published Jonathan Cape, 2004).
- Hans Havlicek (2016) "Clifford parallelisms and planes external to the Klein quadric", Journal of Geometry 107(2): 287 to 303 MR3519950
- Laptev, B.L. & B.A. Rozenfel'd (1996) Mathematics of the 19th Century: Geometry, page 74, Birkhäuser Verlag ISBN 3-7643-5048-2 .
- J.A. Tyrrell & J.G. Semple (1971) Generalized Clifford Parallelism, Cambridge University Press ISBN 0-521-08042-8 .
- Duncan Sommerville (1914) The Elements of Non-Euclidean Geometry, page 108 Paratactic lines, George Bell & Sons
- Frederick S. Woods (1917) Higher Geometry, "Clifford parallels", page 255, via Internet Archive