2 Flats: B Flat Major / G Minor
Two flats is B flat major, or G minor. The flats are B flat and E flat, and the second-to-last of them — B flat — names the key directly.
2 flats
B♭ major / G minor
With flats, the second-to-last flat names the major key directly: the flats run B♭, E♭, so the key is B♭ major.
B♭ major scale
B♭ C D E♭ F G A
G minor scale (relative minor)
G A B♭ C D E♭ F
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B♭ major scale
B♭ C D E♭ F G A
G minor scale
G A B♭ C D E♭ F
The rule, on the first case where it works
With two flats the shortcut becomes usable for the first time: the flats are B flat and E flat, the second-to-last is B flat, and the key is B flat major. The relative minor is three semitones below, on G.
What makes this rule work at all is that in any flat key the fourth-to-last position in the flat order happens to land on the tonic. It is not a coincidence so much as a restatement of the circle of fifths, and once you have checked it on this signature you can apply it to the rest without re-deriving it each time.
The concert band's home key
This is the single most important key signature in wind band music, and the reason is transposition. A B flat trumpet, B flat clarinet, tenor saxophone and soprano saxophone all sound a note called B flat when they play a written C. That means the key that reads as plain C major on their parts is concert B flat.
A composer writing for those instruments therefore has a strong incentive to work in concert B flat, E flat and F: those keys hand the largest section of the ensemble the simplest parts. It is why so much band, brass band and big-band repertoire sits in flat keys while so much guitar and string repertoire sits in sharp ones. Neither tradition is being arbitrary; each is choosing the keys its own instruments read most easily.
G minor
The relative minor here is G minor, and unlike several of the sharp-side relative minors it is genuinely comfortable on most instruments. On a guitar it is a barre chord at the third fret, which is easier than the first-fret barre that F major demands. On strings it uses the open G as its tonic, which gives it a resonance that B flat major itself lacks.
It is also a well-used classical key with a distinctive reputation for intensity — Mozart's Symphony No. 40 is in G minor, and Bach's Fugue in G minor for organ likewise. Both are public domain, and both are good places to hear what the key sounds like at length rather than as a scale.
The scale, and the two flats in it
B flat major is B flat, C, D, E flat, F, G, A. Two notes are lowered relative to C major: the B and the E. Everything else is natural, which keeps the key easy to read even for players who spend most of their time in sharps.
The diatonic chords are B flat, C minor, D minor, E flat, F, G minor and A diminished. Both of the flattened notes are doing structural work here: the B flat is the tonic itself, and the E flat is the third of the fourth-degree chord. Neither is an incidental colour — remove either and the key stops being B flat major.
The seven chords of B♭ major
Every major key produces the same pattern of chord qualities — major on the first, fourth and fifth degrees, minor on the second, third and sixth, diminished on the seventh. Only the names change.
Questions people ask about this signature
- Two flats is what key?
- B flat major, or G minor. The two flats are B flat and E flat.
- Why is so much band music written in B flat?
- Because trumpets, clarinets and tenor saxophones are pitched in B flat: they sound a concert B flat when they read a written C. Concert B flat is therefore the key that gives most of the ensemble the simplest part to read.
- How does the second-to-last-flat rule work here?
- The flats are B flat and E flat in that fixed order. The second-to-last is B flat, and that is the key. The rule works from two flats upward; one flat is the exception.