going to a .012 set (Martin M1700 e.g.) and tuning down to D gives you very close to the same tension as a set of .010s (e.g. Martin M1600) tuned to pitch. therefore, you can put them in, tune down for your voice, get the same feel and play, and have the neck at the same tension...what a deal! the added benefit is that it's eaier to play and easier on the neck joint and bridge.