Guitar Tuner is a tool for tuning classical and acoustic guitars. Some of the features are

* It can listen to instrument sound and measure the sound frequency.

* It can also produce reference sounds when touching the strings on screen.

* you can tune easily by following directions like tighten-loosen

* you can tune by both selecting a string or in auto-selection mode

* there are also "string may be broken" warnings (in manual mode)

* supports more than 50 tuning styles

* callibration between 424-456Hz