<unk>  is the compound described by the formula H3PO3. It is one of the oxoacids of<unk> , other important members being phosphoric acid (H3PO4)  and hypophosphorous acid (H3PO2).  Note that only the reduced<unk>  compounds are spelled with an "ous" ending.  Other names for this acid are orthophosphorous acid and dihydroxyphosphine oxide.
HP(O)(OH)2 is the product of the hydrolysis of its acid anhydride, P4O6:
An analogous relationship connects H3PO4 and P4O10.
Phosphorous acid
Phosphate
Phosphonic acid
Sulfate
Fluoride
Cholesterol
Hypophosphorous acid
Sodium phosphates