In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reactionprogresses in a concerted fashion. pericyclic reactions are usually rearrangement reactions.
If Te describes the reaction (thermal/even) the outcome is Ac (antrafacial or conrotarory).
Te = Ac
If one of the letters of te is incorrect (it is not thermal/even but it is thermal/odd or photochemical/even) the outcome is not given by Ac (the outcome is suprafacial or disrotatory).
If both the letters of Te are incorrect (photochemical/odd) the outcome is given by Ac (antrafacial or conrotarory) - 2 negatives make a positive.