Phosphoric acid units can be bonded together in rings (cyclic structures). The simplest such compound is trimetaphosphoric acid or cyclo-triphosphoric acid having the formula . Its structure is shown in the illustration. Since the ends are condensed, its formula has one less  (water) than tripolyphosphoric acid.
The general formula of a phosphoric acid is , where n is the number of phosphorus atoms and x is the number of fundamental cycles in the molecule's structure; that is, the minimum number of bonds that would have to be broken to eliminate all cycles.
The limiting case of internal condensation, where all oxygen atoms are shared and there are no hydrogen atoms (x = ) is an anhydride , phosphorus pentoxide .