A 12-string acoustic guitar is set up with the strings divided into six “courses” containing two strings each. Each string course consists of a thick (a) and thin (b) string. The thick string in each course is tuned to the same pitch as a 6-string guitar. Most of the thin strings, excluding the 1b and 2b strings, are tuned an octave higher than the thick string in the same course. This gives the 12-string guitar its distinct tone.
Tune the 6a string to the E note. Turn the tuning peg on the headstock to change the pitch of the string. There are two methods to how this can be done. You can play a reference pitch, usually a keyboard, piano, or electric tuner, and adjust the string until it matches the reference. Some electric tuners have a microphone built in for tuning acoustic guitars. The microphone picks up the frequency of the string and displays the note on the screen. Use whichever method you find most comfortable.
Tune the 6b string to the E note one octave higher than the 6a string. Both strings will sound like a single note when played at the same time. The higher octave string gives the 12-string guitar its distinctive tone.
Tune the 5a string to the A note. Then tune the 5b string to the A note one octave higher than the 5a string.
Tune the 4a string to the D note. Then tune the 4b string to the D note one octave higher than the 4a string.
Tune the 3a string to the G note. Then tune the 3b string to the G note one octave higher than the 3a string. Be especially careful not to over tighten this string when tuning it. This will often cause this string to break.
Tune the 2a string to the B note. Then tune the 2b string to the B note. This string, unlike the b string in other courses on the 12-string guitar, is tuned to the exact same pitch as the a string.
Tune the 1a string to the E note. This is the same E note as the high E string of a standard 6-string guitar. Then tune the 1b string to the same E note as the 1a string. This will complete the process to tune a 12-string acoustic guitar.
The strings on a 12-string acoustic guitar are each identified by a number and letter. Each course of strings on a 12-string guitar is numbered. The thinnest, highest pitch course of strings is number 1. Each gradually thicker, lower pitched course is numbered sequentially. The letter indicates if it is the thick (a) or thin (b) string in the course. The 3b string is the most prone to breaking if it is tuned exactly one octave higher than the 3a string. Some guitarists tune the 3b string to the exact same pitch as the 3a string to avoid this. This method of how to tune a 12-string guitar has a minor, but generally not very noticeable change to the sound of the instrument.