Accidentals: sharp, flat and natural signs

Written before a note, a sharp applies to that note — and to the same note in the same octave for the rest of the bar, unless a natural cancels it. In a key signature it applies to every note of that letter, in every octave.

A semitone is the smallest step on the piano: F♯ is the black key between F and G. E♯ and B♯ exist too; with no black key above E or B, they are played on F and C.

It follows the same rules as the sharp: written in the music it lasts to the end of the bar, for that note in that octave; in a key signature it applies throughout.

The sign grew out of a rounded letter b — which is why people often type a lowercase b for it.

A natural can undo an accidental written earlier in the bar, or one from the key signature. In G major, whose signature has F♯, an F with a natural is plain F until the bar line; after it, the key signature applies again.

Editors sometimes print a natural that is not strictly needed, as a reminder after a bar line. That is a courtesy or cautionary accidental, often in brackets.

Why not just write G? Because the letter matters. In G♯ harmonic minor the seventh note is F𝄪; spelling it G would give the scale two Gs and no F. Double sharps keep one letter per scale step.

A natural cancels it completely.

Like the double sharp, it exists for spelling. A diminished seventh chord built on C is C, E♭, G♭, B𝄫: writing the top note as A would break the chord's stack of thirds.

A key signature with one sharp makes every F an F♯, wherever an F appears, until the key signature changes. That saves writing the same accidental again and again, and it points to the key: one sharp is G major or its relative minor, E minor.

The order never varies — sharps F, C, G, D, A, E, B; flats B, E, A, D, G, C, F — and the key signature is repeated at the start of every line of music, unlike the time signature.