E-Z notation, or the E-Z convention, is the IUPAC preferred method of describing the stereochemistry of double bonds in organic chemistry. It is an extension of cis/trans notation that can be used to describe double bonds having three or four substituents.
Following a set of defined rules (Cahn-Ingold-Prelog priority rules), each substituent on a double bond is assigned a priority.
If the two groups of higher priority are on opposite sides of the double bond, the bond is assigned the configuration E (from entgegen, the German word for "opposite").
If the two groups of higher priority are on the same side of the double bond, the bond is assigned the configuration Z (from zusammen, the German word for "together").
| (E)-But-2-ene | (Z)-But-2-ene |
The letters E and Z are conventionally printed in italic type, within parentheses, and separated from the rest of the name with a hyphen. They are always printed as full capitals (not lower case or small capitals), but do not constitute the first letter of the name for English capitalization rules (as in the example above).