A good example of the observed stereospecificity is thermal electrocyclic ring-conversion, i.e., either ring-opening or ring-forming reactions, an important sub-category of pericyclic reactions. Rings of increasing sizes were observed to show alternating stereospecificity: The observed products of the ring-opening of substituted cyclopropyl cations are only consistent with a reaction mechanism in which the two ends of the emerging allyl cation rotate away from each other in a disrotatory fashion, i.e., in clockwise and counterclockwise directions (see Figure for a visualization of disrotatory and conrotatory motions). Conversely, the ring-opening products of substituted cyclobutene are consistent with a conrotatory ring-opening mechanism where the ends of the emerging butadiene compound rotate away from each other in the same (either both clockwise or both counterclockwise) direction. The electrocyclic ring-opening products of substituted 1,3-cyclohexadiene (CHD) are again consistent with a disrotatory mechanism.