Acid anhydrides () have two acyl groups linked by an oxygen atom. If both acyl groups are the same, then the name of the carboxylic acid with the word acid is replaced with the word anhydride and the IUPAC name consists of two words. If the acyl groups are different, then they are named in alphabetical order in the same way, with anhydride replacing acid and IUPAC name consists of three words. For example,  is called ethanoic anhydride and  is called ethanoic propanoic anhydride.