Questions and answers
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A major seventh spans eleven half steps. From C, for example, a major seventh up is B. The number in its name counts the letter names it covers, including both ends: from C to B, C D E F G A B is seven, so it is a seventh. The eleven half steps make it major. With one half step less, C to B♭, it would be a minor seventh.
The major seventh is a half step short of the octave, and played together it’s one of the sharpest sounds on the piano: the two notes almost meet and don’t quite. It is the outer interval of a major 7th chord (C to B in C major 7th), and melodically it’s the distance from the tonic up to the leading tone.
On the keyboard, a major seventh always has ten keys between its two notes. Depending on where it starts, those notes can fall on white or black keys, which gives it three different shapes.
Each shape sits a little differently under the hand, so it is worth knowing them all. Under each keyboard, the same notes are on the staff: played together, then up, then down.
Major sevenths that share this same shape are C–B and F–E. There are ten keys in between (five white, five black).
Major sevenths that share this same shape are D–C♯, E–D♯, G–F♯, A–G♯, and B–A♯. There are ten keys in between (six white, four black).
Major sevenths that share this same shape are D♭–C, E♭–D, G♭–F, A♭–G, and B♭–A. There are ten keys in between (six white, four black).
A seventh goes from a line to a line with two lines in between, or from a space to a space with two spaces in between. Both notes are on lines, or both are in spaces. That stays true whatever the accidentals are: every seventh looks like a seventh, and the accidentals tell us its quality.
This is what makes intervals so useful for reading. Rather than naming every note from the clef, we read from one note to the next: once we know where we are, the shape on the staff tells us how far to move. Reading this way is relational rather than absolute, and it is much faster.
It works the same in the treble and bass clef, and at any height on the staff, so a reader who knows the look of a seventh recognizes it everywhere.
Sevenths are played 1 and 5, with the hand open almost to an octave. It helps to think of the major seventh as an octave with the upper note brought in by a half step.
The spacing on the staff and the spacing in the hand belong together. Beyond a fifth the fingers no longer map one to one onto the notes, so it helps to know the span as a whole hand shape: we see the distance on the staff, and the hand opens to it.
If we move the lower note up an octave (or the upper note down one), a major seventh becomes a minor second: C to B turns into B to C. The numbers of an interval and its inversion add up to nine, and their half steps to twelve (11 + 1). Major becomes minor.
On the keyboard the shapes swap as well: a white-to-black shape inverts to a black-to-white one, and a white-to-white shape stays white to white.
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