To play in a "drop" tuning, simply "drop" your E-string's tuning one whole step to "D". Or go further, down tune your entire guitar so that you're in "Drop C". If you're going to go that far down, it's a good idea to re-string your guitar, as well as get a new setup. With the reduced tension on the neck through the down-tuning you may end up with some warping and problems staying in tune. A good setup, from the beginning can help alleaviate this problem from the start.
Drop D should be: DADG
Drop C should be: CGCF
I may be wrong, but I believe Drop C was/is a popular mandolin tuning. Anyone know anything about this?
