When a trained musician sees a key signature, he or she automatically 'filters' their view of the keyboard to match the key signature. If the key is D major, the white keys F and C are provisionally 'erased' and replaced by the black keys F sharp and C sharp indicated in the key signature.
In the musicians eye, the piano keyboard has become 'key-specific' - it is the keyboard for that key.
This collection of illustrations shows the key-specific keyboard arranged progressing from C major through the sharp keys as far as
F sharp major, then from C major through the flat keys to G flat major.
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Refer back to this sample keyboard illustration as you read the notes below.
The diagrams show first 'the key' in text (here, E major.) Below that is the piano stave (treble and bass clef) showing the key signature sharps or flats. Below that is a list of the sharps or flats in the key signature, in text.
The 'tonic' (home note) is indicated on both the musical stave and the keyboard by the lozenge shapes.
On the demonstration keyboard, the white keys that are played in that key are shown with a strong outline. As they are replaced with the black keys indicated in the key signatures, they are greyed out. The black keys belonging to the key signature illustrated are shown in black; you do not play the greyed-out black keys.
There is one full demonstration octave on each keyboard. Above the keyboard you see the names of the notes in one full octave of the key in question – the scale tones of that key. Below the keyboard is the one-octave scale fingering, the right hand above the left hand.
Classical musicians are usually introduced to key signatures in an overall easy-to-hard/simple-to-complex order, but often without any overall perspective on the build-up of accidentals in the key signatures being given. It is assumed that the logic of key signature progression will somehow 'sink in'.
This ad hoc way of proceeding is not efficient. An overview of key signatures, studied on its own, is easily presented, speeds learning, and puts appreciation of 'key' on a firm footing.
Some valuable understanding of key signature can be absorbed by simply looking through the progressive series of sharp and flat keys, without further explanation. Sharps and flats accumulate regularly and logically as we progress round the circle of fifths - sharps in one direction, flats in the other, and this can be grasped intuitively.
The actual rule behind the accumulation of accidentals (sharps and flats) is here spelled out in detail before the sharp/flat series of keyboards.
The accumulation of sharps
The sharps accumulate as roots fall a perfect fourth/rises a fifth: C major has no sharps, G major (down a perfect fourth/up a fifth) has one sharp, D major (down a fourth/up a fifth again) has two sharps, and so on.
The sharps ‘carry over’ into the new key - once a sharp has appeared in the key signature, it never disappears. The new sharp is required to bring the seventh note of the new scale up a semi-tone to only a semitone below the tonic.
Thus, F sharp is the key signature for G major. The F sharp carries over into two-sharp D major (down a fourth/up a fifth); C sharp is required to sharpen the seventh and form a major scale.
This means that the tonic of a major sharp key is always a semitone above the last sharp in the key signature.
The accumulation of flats
The flats accumulate as roots fall a perfect fifth/rise a fourth: C major has no sharps, F major (down a perfect fifth/up a fourth) has one flat, B flat major (down a fifth/up a fourth again) has two flats, and so on.
As with the sharps, the flats ‘carry over’ into the new key. The new flat is required to lower the fourth note of the new scale a semi-tone to a perfect fourth above the tonic.
Thus, the single B flat is the key signature for F major. The B flat carries over into B flat major (down a fourth); E flat is required to lower the new fourth a semitone and so form a major scale.
This means that the tonic of a major flat key is always a perfect fourth below the last flat in the key signature.
Learn these rules by heart!