https://doi.org/10.1351/goldbook.E01882
The approved @S05976@ of stereoisomeric @A00224@ R1R2C=CR3R4 ( R1 ≠ R2 , R3 ≠ R4); neither R1 nor R2 need be different from R3 or R4), @C01440@ R1R2C(=C=C)n=CR3R4 and related systems e.g. R1R2C=NOH, HON=C{[CH2]n}2C=NOH. The group of highest @C01082@ attached to one of the terminal doubly bonded atoms of the alkene, oxime, etc. or cumulene (i.e. R1 or R2) is compared with the group of highest precedence attached to the other (i.e. R3 or 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 @B00707@ between one and two; and to double bonds involving elements other than carbon. They are not used to describe ring substitution relationships.
See also:
cis-trans isomers