Editing Cahn–Ingold–Prelog priority rules (section)

==== Spiro compounds ====
Some spiro compounds, for example the SDP ligands ((''R'')- and (''S'')-7,7'-bis(diphenylphosphaneyl)-2,2',3,3'-tetrahydro-1,1'-spirobi[indene]), represent chiral, C<sub>2</sub>-symmetrical molecules where the rings lie approximately at right angles to each other and each molecule cannot be superposed on its mirror image.<ref name=":1" /> The spiro carbon, C, is a stereogenic centre, and priority can be assigned a>a′>b>b′, in which one ring (both give the same answer) contains atoms a and b adjacent to the spiro carbon, and the other contains a′ and b′. The configuration at C may then be assigned as for any other stereocentre.