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a. R/S system

(Cahn, R. S., Ingold, C. and Prelog, V., Angew. Chem. Int. Ed. 5 (1966), 385).

The `priority' of an atom is determined by its atomic number. The higher the atomic number the higher the priority. The substituents around a carbon atom are listed in order of their priority. If two or more atoms are identical consider their next substituents, etc. Then look down the bond so that the atom of lowest priority is behind the carbon atom under consideration. If the order of the other substituents going from highest to lowest priority is clockwise the carbon atom is designated R (rectus). If it is anticlockwise it is S (sinister).

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