Hyperconjugation, hybridization, or what? The hyperconjugative effects in primary, secondary, and tertiary alkenes were estimated using ab initio valence bond methods to trace the origin of the empirical Saytzeff rule, which states that the formation of the more substituted alkene is preferred. Hyperconjugation between the π bond and the π-donating substituents governs Saytzeff's rule, whereas all other factors support an anti-Saytzeff pathway.