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1st.- Sestertius, in the masculine singular, signified a single sesterce; and, in order to describe any number whatever of these pieces, the Romans put, with the number, the plural masculine sestertii, thus, centum sestertii, one hundred sesterce pieces. 

2nd.-Sestertium, in the neuter singular, signified mille sestertii, one thousand sesterces; its plural sestertia, with a number, denoted as many thousand sesterce-pieces as that number contained units. Thus, decem sestertia was equivalent to decem millia sestertiorum, ten thousand of sesterces.

3rd.-If the word sestertium was used with the adverbs decies, vicies, centies, millies, etc., centies millies, a hundred thousand, was understood; thus decies sestertium signified decies centies millies sestertiorum, ten times a hundred thousand, or a million of sesterces; centies sestertium was centies centies millies sestertiorum, one hundred times a hundred thousand, or ten millions of sesterces. -- Of this mode of reckoning in sesterces there is an example in the coins of the imperial series: HS NOVIES MILL ABOLITA - See article.

According to some authors, sestertium would here be an adjective referring to mille understood, and would signify a sestertiary thousand; as sestertia would be the adjective of millia, sesteriary thousands; but with the adverbs decies, centies, sestertium would be a contraction of the genitive plural sestertiorum.

When Claudius was elected emperor he gave to each Praetorian soldier sestertia quindena, which (means not fifteen sesterces but) is equivalent to quindena millia sestertiorum, fifteen thousand sesterces.