Mesomeric effect attributed to a substituent in chemical compounds due to overlap of its p- or pi-orbitals with the p- or pi-orbitals of the rest of the molecular entity. The effect is symbolized by M. It occurs in unsaturated organic compounds, where the double bond is separated by a single bond. Examples butadi-1,3-ene, aromatic compounds. The effect is responsible for stability, unusual to unsaturated compounds. It also affects acidity of certain compounds by weakening the inductional effect .