A system for the unambiguous description of stereoisomers used in the R-S convention (see absolute configuration) and in the E-Z convention. The system involves a sequence rule for determining a conventional order of ligands. The rule is that the atom bonded directly to the chiral centre or double bond is considered and the ligand in which this atom has the highest proton number takes precedence. So, for example, I takes precedence over Cl. If two ligands have bonding atoms with the same proton number, then substituents are taken into account (with the substituent of highest proton number taking precedence). Thus, –C2H5 has a higher precedence than –CH3. If a double (or triple) bond occurs to a substituent, then the substituent is counted twice (or three times). An isotope of high nucleon number takes precedence over one of lower nucleon number. Hydrogen always has lowest priority in this system. For example, the sequence for some common ligands is I, Br, Cl, SO3H, OCOCH3, OCH3, OH, NO2, NH2, COOCH3, CONH2, COCH3, CHO, CH2OH, C6H5, C2H5, CH3, H. The system was jointly developed by the British chemists Robert Sidney Cahn (1899–1981) and Sir Christopher Kelk Ingold (1893–1970) and the Bosnian-Swiss chemist Vladimir Prelog (1906–98).