1 flat key signature: which keys use one flat?
A 1 flat key signature belongs to F major and its relative minor, D minor. The single flat is B flat. It sits on the treble third line and the bass second line. One perfect fifth down from C lands on F. The pitches to recompute are 349.23 Hz for F and 293.66 Hz for D.
What a 1 flat key signature prints
F major and D minor share this one flat and stop there. The next flat in the fixed order would be E flat, and those two flats name B flat major, one fifth lower, on the 2 flats key signature page. No flats at all is C major, one fifth higher.
| Flat | Treble staff | Bass staff |
|---|---|---|
| B flat | third line | second line |
| F major | D minor |
|---|---|
| F, G, A, B flat, C, D, E | D, E, F, G, A, B flat, C |
Diatonic triads in F major: F major, G minor, A minor, B flat major, C major, D minor, E diminished. D minor starts on D minor, then E diminished, F major, G minor, A minor, B flat major, C major.
How one flat names F
The shortcut “the major name is the flat second from the right” needs at least two flats. With a single flat there is no second-from-the-right. That one signature is F major. The printed sign is B flat, not F.
Downward fifths: C to F. One step, one flat. Two steps continue to B flat, which is the two-flat signature already linked above.
D minor sits a minor third under F. The prelude on this page is the major one, so the opening harmony leans on F. D minor is the other key that prints the same flat.
Numbers you can check
Equal temperament from A 4 = 440 Hz: pitch = 440 × 2((midi − 69) ÷ 12) Hz. F 4 is midi 65, so 440 × 2(−4 ÷ 12) = 349.228… Hz, written 349.23 Hz. D 4 is midi 62, so 440 × 2(−7 ÷ 12) = 293.665… Hz, written 293.66 Hz.
The accidental count is one, name B flat. One perfect fifth below C spells F. Three semitones under F spell D.
The arithmetic is separate from the recording. On 20 October 2026 the key finder called this F major prelude opening F major at 58% confidence, and it marked that reading low.
A recorded example you can check
Wikipedia’s The Well-Tempered Clavier article, read on 20 October 2026, lists Book 1 No. 11 as Prelude and Fugue in F major, BWV 856. The Open Well-Tempered Clavier score, MuseScore edition by Olivier Miquel, version 1.0 of 19 March 2015, marked Creative Commons Zero, heads the page “Praeludium 11 / BWV 856” and prints one flat on the B line after the treble clef. The clip is the opening of that prelude. The catalog names F major. The engraving shows the one flat. Those are two documents.
On 20 October 2026 at 06:03 EDT this site’s key finder read the clip as F major, Camelot 7B, at 58% confidence, with the hint “Low — check by ear”. The next two candidates were A minor (Camelot 8A, 14%) and D minor (Camelot 7A, 10%). The screen added a low-confidence note: A minor also fits, so check the melody or chords before mixing. The percentage is this file’s estimate, not a measured accuracy rate. The article, the score title, and the finder all name F major. The low mark stays on the page.
What our key tools tell you
The key finder names a key from a recording. Key from chords names one from chord symbols. Neither tool redraws the staff. One flat still means B flat, whether the clip settles on F major or on D minor.
1 flat key signature: quick answers
What key has 1 flat?
F major, and the relative minor D minor. Both use a 1 flat key signature.
What is the 1 flat?
B flat. The order of flats is B, E, A, D, G, C, F, and this signature stops at the first step.
Where does B flat sit?
On the treble staff, the third line. On the bass staff, the second line. The treble place is the one printed on the prelude used here.
What is the relative minor of F major?
D minor, three semitones under F. The recording on this page is the major prelude, not a D minor prelude.
What did the finder return?
F major, Camelot 7B, 58%, marked low. A minor at 14% and D minor at 10% came next.
What this page leaves open
- Harmonic minor in D minor raises the seventh to C sharp. That sharp is not the printed flat.
- 349.23 Hz and 293.66 Hz are equal-temperament arithmetic. They were not measured from the piano file.
- 58% describes this 20-second opening, and the finder marked it low. It does not say that every piece in one flat is F major.