Mandolins are fretted musical instruments, like guitars. Unlike the guitar, however, most mandolins have a movable bridge. The bridge is held in position by the strings. Positioning the bridge is important so that the instrument will play in tune. Making small adjustments to the position of the bridge is called 'adjusting the intonation' of the instrument.
An open string on a stringed instrument touches the instrument at two points. One is the nut, which is the topmost fret, closest to the tuning heads and the 'zero-point' of the string. The other is at the bridge. The distance between these two points is one of the factors determining the pitch of the open string.
- If the bridge is too far from the nut, then fretted notes will play flat relative to the open string.
- If the bridge is too close to the nut, then the fretted notes will play sharp relative to the open string.
Put the bridge in roughly the right position, so that the distance from the nut to the 12th fret is the same as the distance from the 12th fret to the bridge. The bridge should be initially parallel to the nut. Tune up the instrument.
Play a harmonic on the lowest string, by lightly touching your finger against the half-way point on the string, roughly over the 12th fret, and plucking the string. This should produce a note an octave above the open string note.
Now finger the string at the 12th fret and play. This should produce the same note as the harmonic. Listen carefully to the harmonic and to the fingered note. Repeat the notes as often as necessary.
- If the fingered note is flatter than the harmonic, move the bridge closer to the nut.
- If the fingered note is sharper than the harmonic, move the bridge further from the nut.
In order to move the bridge, you may have to slacken all the strings a bit, because it is hard to move the bridge when all the strings are at full tension. Don't let down the strings completely, though, as the strings pull on the neck of the instrument. If this pull is removed, the neck may bend backwards and this bend may damage the neck.
When the bridge is positioned correctly for the lowest string, repeat the process for the highest string. If you have to move the bridge at this stage, turn it so that the point where the lowest string crosses the bridge remains in the same position but the point where the highest string crosses the bridge moves closer to or further from the bridge as necessary.
This may take a bit of fiddling, but is worth doing. Once the bridge is in place, you can tune up and you'll have a mandolin that plays beautifully in tune.