The spin-correlation operators needed to calculate the probabilities o
f the two-electron and six-electron spin alternate arrangements in cat
a-benzenoid hydrocarbons are defined. The projective Huckel wavefuncti
ons were used instead of VB wavefunctions. Bond lengths and local arom
aticity were analysed. A new measure of bond strength and Clar's aroma
tic sextet were obtained by using the spin-correlation order.