E–Z configuration, or the E–Z convention, is the IUPAC preferred method of describing the absolute stereochemistry of double bonds in organic chemistry. It is an extension of cis–trans isomer notation (which only describes relative stereochemistry) that can be used to describe double bonds having two, three or four different substituents.

Following the Cahn–Ingold–Prelog priority rules (CIP rules), we must look at the substituent on each carbon of the double bond and assign a priority to the the two substituents on each separate carbon. For example on the left image below, the methyl on the left of the double bond has a higher priority and it is on the bottom. On the carbon to the right of the double bond the methyl group takes priority over hydrogen and it is on the top. This conformation with one priority substituent being on top and another priority substituent being on the bottom for the other carbon is E notation. When both the priority substituents are on the top for each carbon it is Z notation and vise versa. The E configuration resembles a cis-alkene and the Z configuration resembles the trans-alkene.^[1]

If the two groups of higher priority are on opposite sides of the double bond, the bond is assigned the configuration E (from entgegen, German: [ɛntˈɡeːɡən], 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, German: [tsuˈzamən], the German word for "together").

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