TUNER KORG Rimpitch-C2

To use the Rimpitch, you attach it to the lower side of the sound hole. This means that you'll be able to check the tuning meter at the closest possible distance within your natural angle of vision. Since it's attached in a position where only the performer can casually see it, it will be virtually unnoticed by the audience. This is the ideal tuner for people who don't want to appear to be using a tuner during a live performance.
Since you can leave the Rimpitch attached when stowing your guitar in its case, transportation and storage could not be more convenient. 

The Rimpitch can be installed in round sound holes with a diameter of 100 mm±3 mm, allowing it to fit a wide range of acoustic guitars. There's a hook that keeps it from falling, and since the portion that touches the guitar is made of rubber, there's no danger that your valuable instrument will be scratched.

Scale: 12-note equal temperament; Range (Sine Wave): E2 (82.41 Hz) – E7 (2,637.02 Hz); Precision:+/-1 cent; Reference Pitch:  440 Hz; Dimensions: (W) x (D) x (H) 79 x 32 x 17 mm / 3.11 x 1.26 x 0.67 inches; Battery Life: Approximately 12 hours (Tuner continuously operating, A4 input); Installable Size: Circular sound hole of diameter 100 mm ±3 mm;

It includes a CR1620 lithium battery (3V) x 1 (for verifying operation)