accidentals

Accidentals are signs used in musical notation to alter the pitch of a note from the pitch indicated by the prevailing key signature. The most common accidentals are sharps, flats, and naturals. Sharps raise a pitch by a semitone, flats lower a pitch by a semitone, and naturals cancel the effect of the key signature or any prior accidental for that pitch within the same measure. Double sharps and double flats indicate pitches two semitones away from the natural pitch. In some modern scores, microtonal accidentals specify pitches between the standard semitones.

Scope and interaction with key signatures: An accidental applies to all notes of the same letter name and octave within the current measure. It does not automatically apply to notes in other octaves. A barline resets all accidentals, returning pitches to the key signature defaults for the new measure. A courtesy accidental may be added to prevent ambiguity, especially after ties or in complex passages.

Function and usage: Accidentals create chromatic alteration, enabling melodies and harmonies to move outside the diatonic scale implied by the key signature. They allow temporary departures from the scale, passing tones, leading tones, and mid-phrase tonal colors without changing the underlying key signature. In tonal music, accidentals are typically implied by the harmony, but explicit signs are used for precise pitch control.

Variations and notation: While sharps, flats, and naturals are standard, composers may employ double accidentals or less common signs in early music, modern atonal or microtonal writing, and ensemble scores. Historical practice evolved toward standardized symbols and clear scope within measures.