The degree of advance in the formation of the emerging C-C bond at the transition state for the cycloaddition step of the mechanism of the Wittig reaction dictates the stereochemistry of the final alkenes. Our discovery appears to be of general validity as we tested multiple reactions for stabilized, non-stabilized, and semi-stabilized ylides, holding in every case regardless of the nature of the ylide, the geometry of the associated transition states and the nature of the interactions between substituents.