Two planes that do not intersect are said to be parallel. Two planes specified in Hessian normal
form are parallel iff 
or 
(Gellert et al. 1989, p. 541).
Two planes that are not parallel always intersect in a line.
Parallel Planes
See also
Hessian Normal Form, Parallel, Parallel Lines, Plane, Plane-Plane IntersectionExplore with Wolfram|Alpha
References
Kern, W. F. and Bland, J. R. Solid Mensuration with Proofs, 2nd ed. New York: Wiley, p. 9, 1948.Gellert, W.; Gottwald, S.; Hellwich, M.; Kästner, H.; and Künstner, H. (Eds.). VNR Concise Encyclopedia of Mathematics, 2nd ed. New York: Van Nostrand Reinhold, 1989.Referenced on Wolfram|Alpha
Parallel PlanesCite this as:
Weisstein, Eric W. "Parallel Planes." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ParallelPlanes.html