The steric effect of alkyl groups as characterized by the revised Taft E's parameter is analysed using an approach based on the DARC topological system and its PELCO correlation method. This approach involves an analysis of the systemativ variation of E's in a topological dequencing of alkyl groups and shows the existence of three regions of distinct behaviour: R I, a “normal” behaviour region (ca 6 E's units) in which the contribution of the introduction of successive Me groups to the overall steric effect increases monotonically (groups with 1 to 7 carbons); R II, a region, in which a “levelling” effect is observed, i.e. the contribution diminishes and becomes nil (groups with 8 and 9 carbons); and R III, where this contribution changes sign, “inversion” effect (groups with 10 carbons). Using a series of successive approximations, topological models are developed and tested. The conditions under which the topology may be used to represent the topography (i.e. the real 3-dimensional structure) are considered. The correlation of existing E's values and the reliable prediction of experimentally unavailable steric effects are direct consequences of this treatment.