SUB - Subscript
The SUB element is used for subscripts. Since SUB is inherently presentational, it should not be relied upon to express a given meaning. However, it can be useful for chemical formulas and mathematical indices, where the subscript presentation is helpful but not required. For example:
Chemical formulas include H<SUB>2</SUB>O (water) and C<SUB>21</SUB>H<SUB>27</sub>NO (methadone).
Let <VAR>x</VAR> = <VAR>x<SUB>1</SUB></VAR> + <VAR>x<SUB>2</SUB></VAR> + ... + <VAR>x<SUB>n</SUB></VAR>.