Upasarga is a term used in Sanskrit grammar for a special class of twenty prepositional particles prefixed to verbs or to action nouns. In Vedic, these prepositions are separable from verbs; in classical Sanskrit the prefixing is obligatory.
By the usual rules of euphonic combination the two prepositions ending in visarga, niḥ and duḥ, have the alternative forms nis-/nir- and dus-/dur- respectively. The gaṇapāṭha listing has these variants, not the forms in pausa, and thus has twenty-two items in all.
A versified form of this list may be found in modern primers or textbooks:
upāṅiti viṃsatir eṣa sakhe upasargavidhiḥ kathitaḥ kavinā