An axis about which a set of @[email protected] is held so that it results in a spatial arrangement which is not superposable on its mirror image. For example with an allene abC=C=Ccd the @[email protected] axis is defined by the C=C=C bonds; and with an ortho-substituted biphenyl the atoms C-1, C-1', C-4 and C-4' lie on the @[email protected] axis.