Planet Waves NS Micro Soundhole Tuner
This neat little electronic tuner clamps to the bottom edge of your guitar soundhole. In the playing position, it is very visible to the player and can be turned on or off easily. It has a non-marring clip, a multi-color display and is fast and accurate. The calibration range is 435 to 455. When the string you are tuning is "in tune" the display turns green.