A template that complements {{smallsup}}, i.e. it subscripts <small>-sized text:
{{smallsub|Text to be subscripted}} produces: Text to be subscripted
This template may be listed at Help:Substitution as a good candidate for substituting (i.e. {{subst:smallsub}} ). On a page that uses the template many times, however, this is likely to clutter and lengthen the page significantly.
Template documentation