The terms "conrotatory" and "disrotatory" were coined to describe the relative sense of bond rotation involved in electrocyclic ring opening and closing reactions. When the two ends of the breaking or forming bond rotate in the "same" direction (both clockwise or both counterclockwise — as in the case of the ring opening of 1, 3 or 6 under thermal conditions), the process is termed "conrotatory". When the two ends rotate in "opposing" directions (one clockwise, one counterclockwise — as in the photochemical ring closing of 5), the process is termed "disrotatory". It was found that 4"n-"electron thermal and (4"n" + 2)-electron photochemical electrocyclic reactions were conrotatory in general, while 4"n-"electron photochemical and (4"n" + 2)-electron thermal electrocyclic reactions were disrotatory in general. This pattern was first explained in 1965, when Woodward and Hoffmann proposed the conservation of orbital symmetry (see below) as a key principle that governs the stereochemical course of electrocyclic reactions.