The configuration index is a single digit which is the priority number of the ligand trans to the highest priority ligand. (If there are two possibilities the principle of trans difference is applied). As an example, (acetonitrile)dichlorido(pyridine)platinum(II) complex where the Cl ligands may be trans or cis to one another.<br />
The ligand priority numbers are, applying the CIP rules:
*two chlorides of priority number 1
*acetonitrile priority 2
*pyridine priority 3
In the trans case the configuration index is 1 giving the name(SP-4-1)-(acetonitrile)dichlorido(pyridine)platinum(II).<br />
In the cis case  both of the organic ligands are trans to a chloride so to choose the trans difference is considered and the greater is between 1 and three therefore the name is (SP-4-3)-(acetonitrile)dichlorido(pyridine)platinum(II).