Accidentals: sharps, flats and naturals

The sharp symbol looks like a hash and raises the note a semitone: F sharp sits one fret above F on the guitar and one key to the right on the piano. The flat symbol looks like a lowercase b and lowers the note a semitone, so B flat sits one key to the left of B. Both are written before the notehead but spoken after it — the page says sharp-F, the player says F sharp.

On the keyboard the black keys carry two names each. The key between F and G is F sharp or G flat depending on where the music is going, and choosing between them is not decoration: a scale that already used F will not use F again as G flat, because each letter appears once.

A natural cancels a sharp or a flat and returns the note to its plain letter. It is needed in two situations: when an accidental earlier in the bar is still in force, and when the key signature itself supplies the sharp or flat.

That second case trips people up. In G major every F is sharp because the key signature says so, and writing F natural is the only way to ask for the white key. The natural does not lower the note by definition — it removes an alteration, and whether that sounds lower or higher depends on what it cancels.

A double sharp raises a note by two semitones and is written as a small x. F double sharp sounds the same as G, but it is not spelled G — it appears where the harmony demands an F of some kind, typically as the leading note of G sharp minor. A double flat, written as two flat signs, lowers by two semitones in the same way.

These signs look exotic and follow the same rule as everything else: they exist so that each letter keeps its place in the scale. Renaming F double sharp as G would put two Gs in a row and one letter missing, and the chord on the page would stop matching the chord in the ear.

An accidental applies from where it appears until the end of that bar, and only on its own line or space. The next bar line cancels it, unless a tie carries the note across, in which case the tied note keeps the alteration and the next struck note does not.

Editors often add a courtesy accidental in brackets when a note might be misread — after a bar full of sharps, for instance. It changes nothing musically and is there purely so the player does not hesitate.

The key signature stands at the start of every staff and applies to the whole piece until it changes. An accidental stands inside the bar, applies to one note, and expires at the bar line. The two describe the same alteration and differ only in reach.

A piece in F major has one flat in the key signature and therefore no B flats written inside the bars — they are already implied. When a B natural does appear, it means something: usually a temporary move away from the home key, and it is worth hearing rather than just executing.