In inorganic chemistry,<unk>  is a phosphorus oxoacid with a formula of H3PO3, more commonly known as phosphorous acid.  It exists in solution as two tautomers, the major one being HP(O)(OH)2 and the minor one P(OH)3.  The former is sometimes termed<unk> , with the latter designated as phosphorous acid.  Sometimes confusingly, both these names are also used to refer to H3PO3 in general, i.e. both tautomers.
In organic chemistry, a<unk>  is a compound with the general formula RP(O)(OH)2.
An example of an organic<unk>  is Foscarnet.
An oligophosphonic acid refers to a few molecules of<unk>  condensed into a molecule with the loss of water.
A general formula for such oligophosphonic acids is (HPO)nOn-1(OH)2, where n = 2, 3, 4, etc., oligo-. A polyphosphonic acid can have dozens of such<unk>  units condensed in a row with the loss of H2O for each unit added on.
An example that incorporates triphosphonic acid: ethane-1,1,2-triphosphonic acid. In some<unk>  anhydrides (RPO2)3, R can be tBu, 2-methylphenyl, 2,4,6-trimethylphenyl.
Diphosphonic acid
Tautomers of H3PO3:phosphonic acid (left)phosphorous acid (right)
The general structure of an organic<unk> 
Diphosphonic acid
Tautomers of H3PO3:phosphonic acid (left)phosphorous acid (right)
The general structure of an organic<unk>
Phosphonic acid
Phosphate
Phosphorous acid
Sodium phosphates
Glycerin
Sulfate
Amino acid
Calcium phosphate