Tuning the guitar is one of the most important things to do every time you play your instrument. With the advent of smart phone apps gone are the days of carrying around a physical tuner. For beginners I recommend the app GuitarTuna because of its easy to use interface. For advancing players I like Cleartune. While GuitarTuna isn't quite as accurate as Cleartune, it is typically easier to use for beginners.

GuitarTuna on Browser
GuitarTuna on iOS
GuitarTuna on Google Play

Cleartune on iOS
Cleartune on Google Play

Strobe tuners

I'm also a big fan of super accurate strobe style tuners that not only allow you to tune up but which can also be used to set intonation. When playing live I like to use either the TC Electronic Polytune Clip or a Sonic Research Turbo Tuner. These tuners are super useful for noisy environments where your phone microphone will struggle with background sounds.

Polytune Clip Amazon Link

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Sonic Research Turbo Tuner