A theoretical space used in the mathematical analysis of crystals in which there is a lattice called the reciprocal lattice. If the primitive translation vectors of the real (direct) lattice are a, b, c the primitive translation vectors a′, b′, c′ of the reciprocal lattice are defined by a′=b × c, b′=c × a, c′=a × b. This definition means that every plane in a real lattice becomes a point in the reciprocal lattice. The concept of reciprocal space is very useful in X-ray crystallography and energy-band theory.