Using this notation, Woodward and Hoffmann state in their 1969 review the general formulation for all pericyclic reactions as follows:
 and (4r) components is odd.</blockquote>
Here, (4q + 2) and (4r) refer to suprafacial (4q + 2)-electron and antarafacial (4r)-electron components, respectively.  Moreover, this criterion should be interpreted as both sufficient (stated above) as well as necessary (not explicitly stated above, see: if and only if)