Crystal structural studies of several linear and branched triangulanes as well as spiro-cyclopropanated bicyclopropyridines by x-ray diffraction reveal significant differences between proximal and distal bond lengths in the 3-membered rings. Combined with previously published structural data, a general additivity scheme describing the geometry of such hydrocarbons was elaborated. The p donor spirocyclopropane and a double bond shorten the proximal bonds by 0.0234 and 0.0327 .ANG. and lengthen the distal bonds by 0.0227 and 0.0327 .ANG. with respect to the unperturbed value of 1.5008 .ANG.. The results of these calcns. were compared with those obtained by semiempirical and ab initio methods and explained qual. by using the fragment MO (FMO) approach.