Notes and semitones, and why seven names are enough

A piano has twelve keys to the octave. Western music uses seven letters to name them. That gap, twelve sounds and seven names, is not a historical oversight: it is what makes it possible to write music that can be read.

The twelve sounds

From one C to the next there are twelve equal steps. Each step is a semitone, the smallest distance the system recognises. Two semitones make a whole tone.

Look at the layout of the keyboard. The black keys are not scattered at random: there are two, then three, then two again. And between E and F, and between B and C, there is no black key at all.

The seven names

The seven letters, C D E F G A B, or do re mi fa sol la si, name the white keys. The distances between them are not all the same:

• C→D, D→E, F→G, G→A, A→B: a whole tone each
• E→F and B→C: a semitone each

Those two semitones in the middle of the series are the source of almost everything that follows. They are the reason the major scale has the shape it has, and the reason the diatonic field has some major chords and some minor ones.

Accidentals

To name the five remaining keys, a letter is altered:

• Sharp (♯) raises by a semitone
• Flat (♭) lowers by a semitone
• Natural (♮) cancels the alteration

And here is the point that confuses beginners most: C♯4 and D♭4 are the same key. Two spellings, one sound.

If they sound the same, why spell them differently?

Because the spelling does not describe the sound; it describes the role the note plays.

In D major, the third note is F sharp. Writing it as G flat would produce a scale with two Gs and no F, and the staff would become unreadable: every line and every space of the staff corresponds to a letter, and using the same letter twice throws everything out of line.

The rule that decides is simple and has no exception in major and minor scales: each letter appears exactly once. Once the letter is chosen, the accidental comes with it; there is no choice.

Doubles

If each letter has to appear once, sometimes one of them needs two accidentals. In the harmonic minor scale of G♯ minor, the leading tone is F double sharp: the same key as G, but the letter G is already taken by the first degree.

It looks like overkill, and it is exactly the opposite: it is the system refusing to lie about which degree is which.

What to take from this

Twelve sounds, seven letters. The letter comes from the role the note plays in the scale; the accidental comes afterwards, to get the sound right. Whoever inverts that order, choosing the sound first and the letter after, ends up writing C sharp where D flat should be, and loses the information that makes the score make sense.