Given non-coplanar vectors a, b, c, their reciprocal system a', b', c' is defined as: a' = (b x c)/[a b c], b' = (c x a)/[a b c], c' = (a x b)/[a b c]. Properties: a.a' = b.b' = c.c' = 1 and a.b' = a.c' = 0 (and similarly for other cross pairs). The reciprocal of the reciprocal system gives back the original system. This concept appears occasionally in JEE but is useful for resolving vectors along non-orthogonal axes.