In a few cases, the prefixes "ortho-" and "para-" occur in names of some oxyacids and their derivative anions. In such cases, the "para-" acid is what can be thought as remaining of the "ortho-" acid if a water molecule is separated from the "ortho-" acid molecule. For example, phosphoric acid, H3PO4, has sometimes been called "orthophosphoric acid", in order to distinguish it from metaphosphoric acid, HPO3. However, according to IUPAC's current rules, the prefix "ortho-" should only be used in names of orthotelluric acid and orthoperiodic acid, and their corresponding anions and salts.