Note: For the hydrogen isotopes protium, deuterium, and tritium, the nuclide symbols , , and , respectively, are used. The symbols D and T for and , respectively, may be used, but not when other modifying nuclides are also present, because this may cause difficulties in alphabetic ordering of the nuclide symbols in the isotopic descriptor. Although the symbols d and t have been and are still used in place of and in names formed according to the Boughton system (see R-8.0), in no other cases are lower-case letters used as atomic symbols. Therefore, the use of d and t in chemical nomenclature outside if the Boughton system is not recommended.
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