In the mid 60's, R.B. Woodward and Roald Hoffmann, among others, advanced a set of rules based on molecular orbital symmetry, to predict the direction of molecular rearrangements. The difficulty in providing strong support for these rules arises from the difficulty in proving, or even adequately defining the prerequisite condition of concertedness. This review offers the chemical behavior of main group IV substituted cyclic polyenes as evidence for the operation of symmetry predictions, with a regard for concertedness. In particular, the author's observations with triphenyl cycloheptatrienyl stannane are offered to reject the previous use of "Least Motion" predictions in favor of symmetry arguments. © 1974.