Because, as Foolestroupe has mentioned, of temperament issues, only perfect fifths, unisons and octaves should be used for tuning. Fortunately, mandolins are tuned in fifths so this is no problem.

First, of course, your bridge must be in the right place. That is the harmonic a the twelfth fret must exactly match the fretted note at the twelfth fret. If that isn't so, your mandolin will never play in tune.

I keep a digital tuner in my mandolin case to use in loud situations but it's really a second choice. To begin, be sure each string is within a semitone or so of correct pitch. This is so the instrument will already have distributed the string tension and you won't have to spend time retuning strings you've already tuned.

If you are in a situation where you can hear to tune you want to tune the A string to an audible pitch reference such as a good quality tuning fork or the A reference tone on your electronic metronome. Listen for the "beat," the wavy sound you get when two sounds are close but not exactly together. Tune the beat completely out of the third string and the tune the fourth string to match it.

Now, making sure the first string is slightly flat rather than sharp, play the first and third strings together and listen for the beat. Even though they are not the same note, you will hear a beat as they approach the perfect fifth interval. Tune the beat to "zero" as before then tune the second string to match the first.

Repeat this procedure with the fifth and sixth pairs using the A string as a reference. The G string cannot be tuned against the A string in this way but must be tuned against the D string so make certain the D string is correct.

When all the strings have been tuned, you can strike a harmonic on the G string at the fifth fret and it should match the G note played on the E string at the third fret. Don't use this harmonic to tune, only as as rough check on the whole process. If it's off, check the perfect fifth intervals between adjacent pairs again. If they are correct, your bridge may not be in the right place after all. Either that or your fretboard is incorrect.

      - Mark