IUPAC, however, recommends against calling future compounds not yet discovered with a name ending with the word "acid". Indeed, acids can be called with names formed by adding the word "hydrogen" in front of the corresponding anion; for example, sulfuric acid could just as well be called "hydrogen sulfate" (or "dihydrogen sulfate"). In fact, the fully systematic name of sulfuric acid, according to IUPAC's rules, would be "dihydroxidodioxidosulfur" and that of the sulfate ion, "tetraoxidosulfate(2−)", Such names, however, are almost never used.