Since stereochemistry is a three-dimensional science which requires twodimensional representation on paper, a number of conventions for representing structures has been developed. The physical and chemical properties of enantiomers are identical in a symmetrical environment but differ in a dissymmetric environment. In particular, enantiomers rotate the plane of plane-polarized light in opposite directions, although to the same extent per mole in the same conditions: they are described as being optically active. The mechanisms of epimerization parallel those of racemization. Epimerization is said to occur when there is a change in configuration at one stereocentre in a compound which possesses more than one such centre. The absolute configurations of other optically active compounds can be obtained from that of the (+)-tartrate by correlative methods. Optical isomerism occurs also in suitably substituted polyphenyls but the stereochemistry is more complex since both meso forms and geometrical isomerism are also possible.