Is it really free
Yes, and open source under the MIT licence. There is no paid tier, no account, and no upsell for the report. The tools it is measured against cost several hundred euro, and the report module is usually sold separately.
Does it work offline
Entirely. Nothing is sent anywhere. The only thing that touches the network is the download page on this site.
What can it tune
Free-reed instruments. Piano accordions, chromatic and diatonic button boxes, and the same principles cover concertinas, bandoneons, harmonicas, melodicas and reed organs. Anything where the pitch comes from a steel tongue over a slot.
Will it tune a chromatic button box
Yes. And a bisonoric one - tick "the bellows change the note" on the instrument and the app treats push and pull as the different notes they are.
Do I need a special microphone
No, and this is the thing people overspend on. You are measuring frequency, not recording an album, and a cheap dynamic microphone measures frequency exactly as well as an expensive condenser. What matters is placement - same side as the reeds, under half a metre - and turning off any echo cancellation or noise suppression in the microphone's driver. Those deform the signal and no tuner can measure a deformed signal.
How accurate is it
The engine resolves frequency and beat rates far below anything the ear can detect, across E0 to C9, with a reference pitch adjustable from 430 to 450 Hz. In practice the limits you meet are physical, not numerical: a beat slower than about 0.6 Hz is indistinguishable from bellows wobble, and no tuner can honestly report one.
The accuracy claims are backed by a test suite that runs real recordings and synthetic signals through the engine on every change. You can run it yourself; it is in the repository.
Why does it sometimes refuse to show a number
Because it does not know it. When two reeds are too close to separate, what comes back are lobes of one peak - not a reed each. Numbers written against them would send you to file a reed that is not there. So it prints the beat, which is the honest reading, and no per-reed figures.
A missing number costs nothing. A confident wrong one costs a reed.
Can I tune by ear and use this as a check
That is what a lot of people do, and the app is built for it. It is why the beating curve can be fitted to a recording of your own instrument rather than typed: you are making the tremolo consistent across the keyboard, not imposing anyone else's numbers on it.
Does it ship the standard musette tunings
No, deliberately. There is no library of French / Italian / Slovenian curves, because shipping one would mean the app assuming an answer to the only question you are actually asking. Record the instrument and let it fit the curve the instrument already has, or type your own.
Do I have to create a session
No. Open the tuner, check a note, leave - nothing is created, nothing saved, nothing asked. A session is an upgrade you opt into for a job worth keeping.
Can I export the data
Reports export as HTML (prints on A4) and CSV. Sessions and instruments are files you already own, and import and export whole.
Can it read Dirk's .btg and .tun files
Not currently. They are undocumented binary formats and there are no sample files on hand to test against. It is worth attempting only for someone with a real library to bring across.
Does it measure chords, or several octaves at once
Not yet. It resolves up to three reeds within one note, which is what a musette right hand needs and what the whole record-and-file workflow runs on. Chord identification and a user-facing octave mode are not written. A bass button sounding four or five octave voices at once is planned and is the reason the data model already carries more reeds than the tuner currently splits.
What platforms
Linux is home turf: AppImage, deb and rpm. Windows has an installer. macOS is written but not built - it needs Mac hardware to compile and notarise on, so there is nothing to download yet.
What languages
English and Polish, and the printed report follows whichever you set - decimal commas included. The CSV export is deliberately left in English with a decimal point: it is read by a spreadsheet, not a person, and its headings are field names.
Note naming covers international (C D E F G A B), German (C D E F G A H, where B means B flat), Polish (C Cis D Dis E F Fis G Gis A Ais H) and solfege.