TuneHaven What A440 means, and when to change it
Concert pitch puts the A above middle C at 440 Hz, and everything else follows from it. When you would deliberately use 442, 415 or something else, and what happens if you get it wrong.
Every frequency on every tuning chart comes from one number. Fix the A above middle C at 440 Hz, and equal temperament determines all the others: multiply by the twelfth root of two to go up a semitone, divide to go down.
That reference is called concert pitch, or A440. It is what a tuner assumes unless you tell it otherwise.
Why it is adjustable at all
Because 440 is a convention, not a law of physics, and several groups of musicians do not use it.
Orchestras often run sharp. Many European orchestras tune to 442 or 443 Hz, some higher still. The usual explanation is that a slightly higher pitch sounds marginally brighter and more brilliant, and once one ensemble does it the others follow. If you are joining an orchestra, ask what they tune to rather than assuming.
Historical performance runs flat. Baroque ensembles commonly use A415, which is almost exactly a semitone below 440. Gut strings and period instruments were built for lower tension, and the repertoire sits differently. A modern instrument can usually be taken down to 415 without harm, though it will feel slack.
Sometimes you match a recording. Older recordings, and anything that has been sped up or slowed down in production, can sit anywhere. If you are playing along with a track and everything sounds subtly wrong at 440, the track is probably not at 440.
Sometimes you match a stubborn instrument. A piano that has drifted, or is deliberately tuned low, is not going to move for you. Set your reference to match the piano and tune to that.
Setting it in TuneHaven
A4 calibration is in the tuner settings and it is free, along with every other setting. Change the reference and every target across every instrument shifts with it, so all the presets stay internally consistent.
The setting persists, which cuts both ways. If your tuner has been reading strangely for a week, check that you did not leave the reference somewhere unusual.
What goes wrong if the reference is off
Nothing, as long as you are alone. Your instrument is in tune with itself and everything sounds fine.
The moment you play with anyone else, it falls apart. Two guitars a couple of hertz apart produce a slow beating on every unison, and the whole thing sounds queasy without either player being able to say why. Against a piano or any other fixed-pitch instrument, you are simply wrong and it is not.
So: match your reference to whatever cannot change. A piano or an organ wins. A recording wins. Between two guitars, agree and both set the same number.
The relationship to temperament
A440 sets where the notes are. Temperament sets the spacing between them. They are separate choices, and TuneHaven exposes both.
Equal temperament, the default, divides the octave into twelve mathematically identical steps. It is what a piano uses and what almost everyone means by “in tune”. The temperament guide covers the alternatives and why a string player tuning pure fifths ends up disagreeing with a keyboard.