Strain is present in a molecular entity or transition structure if the energy is enhanced because of unfavourable bond lengths, bond angles or dihedral angles ('torsional strain') relative to a standard. It is quantitatively defined as the standard enthalpy of a structure relative to a strainless structure (real or hypothetical) made up from the same atoms with the same types of bonding. (The enthalpy of formation of cyclopropane is 53.6 kJ mol-1, whereas the enthalpy of formation based on three 'normal' methylene groups, from acyclic models, is -62 kJ mol-1. On this basis cyclopropane is destabilized by ca. 115 kJ mol-1 of strain energy.)