Key Signatures with Flats
Let's finish with flat key signatures. You'll learn to read one to find the key, and to write the key signature of a given key.
Like sharps, the flats of a key are placed right after the clef, in their order, on the lines or spaces of the notes they alter. They form a flat key signature, and their effect lasts for the whole piece, in every octave, unless the key signature changes.
From key signature to key
The last flat always falls on the subdominant. So the tonic is the note a perfect fourth below the last flat.
- With one flat, Bb, the tonic is F, a perfect fourth below.
- With five flats, the last one Gb, the tonic is Db, a perfect fourth below.
The next-to-last flat rule
Even simpler: the next-to-last flat in the key signature names the tonic. With four flats, Bb, Eb, Ab and Db, the next-to-last is Ab, so the key is Ab major.
This follows from the fact that flats are added by rising fourths: if the last flat is the subdominant, the tonic lies a fourth below it, exactly where the next-to-last flat sits.
From key to key signature
The last flat falls on the 4th degree. For example, take the tonic Db. The last flat is a fourth above it, Gb. In the order of flats, Gb is the 5th, so there are five flats in the key of D flat: Bb, Eb, Ab, Db and Gb.
Method: with sharps, the major tonic is a half step above the last sharp; with flats, it is the second-to-last flat (F major for one flat). The relative minor is a minor third lower.
1. Find the keys indicated by key signatures of 2, 3, 6 and 7 flats.
2. Give the key signatures of F, C flat, E flat and G flat major.
Before moving on, check that flat key signatures are clear with these few questions.