12th-fret check
A string in tune can still play out of tune higher up the neck. The 12th-fret check tells you whether a string's saddle should move, which way, and on a guitar about how far.
Why the 12th fret
The 12th fret sits halfway along the string, so a string pressed there should sound exactly one octave above the open string. If it sounds higher, the part of the string from the fret to the saddle is too short; if lower, too long. Setting this right is called intonation, and it is done by moving the string's saddle, the point on the bridge where the string rests.
Check a string
- Tap what you are tuning at the top of the tuner (Guitar · Standard). At the bottom of What are you tuning?, tap 12th-fret check.
- Tap the string you want to check. Tune it first: Twangle asks you to if it is more than 10 cents off.
- Play the open string and let it ring, until the ring round the note fills, about a second.
- Now press the string down at the 12th fret and play it again. Press it as you would to play a note: a light touch gives a harmonic, which always sounds the exact octave, whatever the saddle.
- Twangle tells you what to do: Fine, Move the saddle back (away from the neck) or Move the saddle forward (towards the neck), with how many cents the 12th fret is off.
Then Next string, or Check again after you moved the saddle. Each string you checked keeps a small mark on its button, and when you are through, Twangle says how many saddles to move. To go back to the tuner, tap the small × in the corner of the check, or Back.
How far to move it
On a guitar Twangle also says about how far, to the nearest half millimetre, for example Away from the neck, about 3 mm. It counts on a guitar's usual length of string, about 65 cm; on a guitar with a shorter or longer neck, an answer of 3 mm is off by less than a fifth of a millimetre. On a bass, ukulele or mandolin it gives the direction only: their lengths vary too much to say.
A string counts as fine within 3 cents of the octave, about a millimetre of saddle on a guitar.
Before you move a saddle
- Check with fresh strings, and after any change to the string height or neck: old strings and a new setup both change the intonation.
- Loosen the string a little before you move its saddle, then tune it again and check once more.
- On an electric guitar each string's saddle has its own screw. On most acoustic guitars the saddle is one strip of bone or plastic set into the bridge; changing it is a job for a guitar repairer.
The check is there for guitars (the 12-string too), basses, the ukulele and the mandolin. The violin, viola and cello have no frets, and on the banjo the short fifth string has its octave at the 17th fret.
For the curious: what Twangle compares
Twangle waits until each note is steady, so the pluck and the dying end don't count, then takes the middle value of what it hears for about a second. It compares only the two notes with each other, so it works at any reference pitch, and a string a few cents out of tune still gets the right answer.
A fretted note that is more than half a semitone from the octave isn't the 12th fret at all (another fret, another string, or the open string again), so Twangle says That wasn't the octave instead of telling you to move a saddle 20 mm. On a guitar, moving the saddle one millimetre changes the 12th fret by about 2.7 cents.