Term used to refer to stereoisomerism resulting from the non-planar arrangement of four groups in pairs about a chirality axis. It is exemplified by allenes abC=C=Ccd (or abC=C=Cab) and by the atropisomerism of ortho-substituted biphenyls.
The configuration in molecular entities possessing axial chirality is specified by the stereodescriptors Ra and Sa (or by P or M).
