Relative tuning is so named because you don't need any outside reference to which you tune a guitar. As long as the strings are in tune in a certain relationship with each other, you can create sonorous and harmonious tones. Those same tones may turn into sounds resembling those of a catfight if you try to play along with another instrument, however; but as long as you tune the strings relative to one another, the guitar is in tune with itself.
To tune a guitar using the relative method, choose one string as the starting point — say, the 6th string. Leave the pitch of that string as is; then tune all the other strings relative to that 6th string by using the fifth-fret method.
The fifth-fret method derives its name from the fact that you almost always play a string at the fifth fret and then compare the sound of that note to that of the next open string. You need to be careful, however, because the fourth fret (the fifth fret's jealous understudy) puts in a cameo appearance toward the end of the process.
Here's how to get your guitar in tune by using the fifth-fret method:
1. Play the fifth fret of the 6th (low E) string (the fattest one, closest to the ceiling) and then play the open 5th (A) string (the one next to it).
Let both notes ring together. Their pitches should match exactly. If they don't seem quite right, determine whether the 5th string is higher or lower than the fretted 6th string. If the 5th string seems lower, or flat, turn its tuning key with your left hand to raise the pitch. If the 5th string seems sharp, or higher sounding, use its tuning key to lower the pitch. You may go too far with the tuning key if you're not careful; if so, you need to reverse your motions. In fact, if you can't tell whether the 5th string is higher or lower, tune it flat intentionally (that is, tune it too low) and then come back to the desired pitch.
2. Play the fifth fret of the 5th (A) string and then play the open 4th (D) string.
Let both of these notes ring together. If the 4th string seems flat or sharp relative to the fretted 5th string, use the tuning key of the 4th string to adjust its pitch accordingly. Again, if you're not sure whether the 4th string is higher or lower, "overtune" it in one direction — flat, or lower, is best — and then come back.
3. Play the fifth fret of the 4th (D) string and then play the open 3rd (G) string.
Let both notes ring together again. If the 3rd string seems flat or sharp relative to the fretted 4th string, use the tuning key of the 3rd string to adjust the pitch accordingly.
4. Play the fourth (not the fifth!) fret of the 3rd (G) string and then play the open 2nd (B) string.
Let both strings ring together. If the 2nd string seems flat or sharp, use its tuning key to adjust the pitch accordingly.
5. Play the fifth (yes, back to the fifth for this one) fret of the 2nd (B) string and then play the open 1st (high E) string.
Let both notes ring together. If the 1st string seems flat or sharp, use its tuning key to adjust the pitch accordingly. If you're satisfied that both strings produce the same pitch, you've now tuned the upper (that is, "upper" as in higher-pitched) five strings of the guitar relative to the fixed (untuned) 6th string. Your guitar's now in tune with itself.
You may want to go back and repeat the process, because some strings may have slipped out of tune.
When you tune in the normal way, you use your left hand to turn the tuning peg. But after you remove your finger from the string that you're fretting, it stops ringing; therefore, you can no longer hear the string you're trying to tune to (the fretted string) as you adjust the open string. However, there's a way to tune the open string while keeping your left-hand finger on the fretted string. Simply use your right hand! After you strike the two strings in succession (the fretted string and the open string), take your right hand and reach over your left hand (which remains stationary as you fret the string) and turn the tuning peg of the appropriate string until both strings sound exactly the same.