Acid anhydrides form when the OH- groups from two acids react forming an anhydride bond and water:

Two carboxylic acids:
  acetic acid with acetic acid
 See jmol: acetic anhydride

A carboxylic acid and phosphoric acid:
 acetic acid and phosphoric acid
 See jmol: acetyl phosphate

Two phosphates:
dCMP and phosphoric acid
 See jmol: dCDP