Yeah, Xeno's right. I tuned my main axe, an ESP with a locking floyd rose trem system, down to C, but the music store I was at to buy strings didn't have anything past 11 gauge. I ended up getting a set of hybrid dean markeley's, with heavier gauge strings on the bottom and lighter gauge on the higher strings for easier soloing. Unfortunately, the gauge was way too light for C, so I ran into the problem of fretting out past the 19th fret on my G-B-e strings, or Eb-G-c in C tuning anyways. Make sure you get your guitar adjusted appropriately for the tuning, as your going to be putting a lot more stress on the neck then its used to.