The approved stereodescriptors of stereoisomeric alkenes $\ce{R1R2C=CR3R4}$ ($\ce{R1} \neq \ce{R2} , \ce{R3} \neq \ce{R4}$); neither $\ce{R1}$ nor $\ce{R2}$ need be different from $\ce{R3}$ or $\ce{R4}$), cumulenes $\ce{R1R2C(=C=C)_{n}=CR3R4}$ and related systems e.g. $\ce{R1R2C=NOH}$, $\ce{HON=C\{[CH2]_{n}\}2C=NOH}$. The group of highest CIP priority attached to one of the terminal doubly bonded atoms of the alkene, oxime, etc. or cumulene (i.e. $\ce{R1}$ or $\ce{R2}$) is compared with the group of highest precedence attached to the other (i.e. $\ce{R3}$ or $\ce{R4}$). The stereoisomer is designated as $Z$ (zusammen = together) if the groups lie on the same side of a reference plane passing through the double bond and perpendicular to the plane containing the bonds linking the groups to the double-bonded atoms; the other stereoisomer is designated as $E$ (entgegen = opposite). The descriptors may be applied to structures with a fractional bond order between one and two; and to double bonds involving elements other than carbon. They are not used to describe ring substitution relationships.