While using an electronic tuner is the most accurate way to tune a guitar, you might not have one around when you need it. Another possible dilemma is if the batteries in your tuner die right before a gig. In these instances, it is wise to have a back-up plan, such as tuning your guitar using a harmonica as a reference point.
Play an E on your harmonica. With a C harmonica (the most common type of harmonica), play this note by blowing into the eighth hole. (In numbering the holes, always move from left to right.)
Pluck the high E string of your guitar. This is the thinnest string and is located closest to the floor when you are playing the instrument.
Tighten the tuning key that corresponds with the high E string if the note sounds lower than the E on the harmonica. Loosen the tuning key if the guitar's note sounds higher than the note on the harmonica.
Continue to make minor adjustments with the tuning key until the high E string sounds like the high E on the harmonica.
Use the same process to tune the other strings.
Tune the next string (B string) to the note that is produced by inhaling ("drawing") into the harmonica's seventh hole.
Tune the third string (G string) to the note that is produced by blowing on the harmonica's sixth hole.
Tune the fourth string (D string) to the note that is produced by drawing on the harmonica's fourth hole.
Tune the next string (A string) to the note that is produced by drawing on the harmonica's sixth hole. Although this note is an A, it will sound higher than the guitar's A. Still, it can be used as a reference point.
Tune the thickest string (low E) to the note that is produced by blowing on the harmonica's second hole.