H₃PO₄, phosphoric acid, can be written O=P(OH)₃. It has three acidic protons (attached to oxygen) and is therefore tribasic. Its salts are called phosphates.

But H₃PO₃, phosphonic acid, has one of the hydrogens directly bound to phosphorus as O=PH(OH)₂. This means that only two hydrogens are available to act as acids. Its salts are called phosphonates.

There is also an H₃PO₃, phosphorous acid P(OH)₃, that is tribasic. Its salts are called phosphites.

Dan Berger