Sketch of the toroidal mapping of a doubly periodic volume. In the mapping, surface ﹩f1﹩ is joined to surface ﹩f6﹩ while surface ﹩f2﹩ is joined to surface ﹩f5﹩. If no flux penetrates the boundaries of the toroidal volume, then the helicity is simply ﹩\boldsymbol{A}{\boldsymbol{\cdot}} \boldsymbol{B}﹩ integrated over the volume.