It is often found necessary, in order to expand the normal range of the guitar, to tune the sixth string @ one tone lower to D (called D tuning) and, less commonly along with the fifth string % one tone lower to G (called G tuning).
Once either of these strings is tuned down it is, of course, necessary to play all fretted notes two frets higher than normal. This modification usually occurs, naturally enough, for pieces in the keys of D and D minor (D tuning) and G and G minor (G tuning).