A rearrangement is also a two component pericyclic reaction: one component is the π-system, the other component is the migrating group. The simplest case is a [1,"j"-hydride shift across a π-system where "j" is odd. In this case, as the hydrogen has only a spherically symmetric s orbital, the reaction must be suprafacial with respect to the hydrogen. The total number of electrons involved is ("j" + 1) as there are ("j −" 1)/2 π-bond plus the σ bond involved in the reaction. If "j" = 4"n" − 1 then it must be antarafacial, and if "j" = 4"n" + 1, then it must be suprafacial. This agrees with experiment that shifts are generally not observed as the symmetry-allowed antarafacial process is not feasible, but [1,5-hydride shifts are quite facile.