Hydrogen (H) has naturally occurring isotopes (1H, 2H, and 3H). The first two are stable, while 3H is radioactive. Hydrogen is the only element whose isotopes have different names that are in common use today. The 2H isotope is usually called deuterium, while the 3H isotope is usually called tritium. The symbols D and T are sometimes used for deuterium and tritium (instead of 2H and 3H). Hydrogen isotopes, especially the deuterium of methane combined with the carbon isotopes are commonly used in the geochemical analyses of natural gas to provide information about the genetic origin and maturity of the gas.