acid anhydrides

Compounds consisting of two acyl groups bonded to the same oxygen atom acyl-O-acyl. Symmetric and mixed anhydrides have identical and different acyl groups, respectively. E.g.
A00072-1
acetic anhydride,
A00072-2
acetic benzenesulfonic anhydride,
A00072-3
(thiobenzoic) anhydride.
Source:
PAC, 1995, 67, 1307 (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1311