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In music, an accidental is a note of a pitch (or pitch class) that is not a member of the scale or mode indicated by the most recently applied key signature. In musical notation, the sharp (♯), flat (♭), and natural (♮) symbols, among others, mark such notes—and those symbols are also called accidentals.
In the measure (bar) where it appears, an accidental sign raises or lowers the immediately following note (and any repetition of it in the bar) from its normal pitch, overriding the key signature. A note is usually raised or lowered by a semitone, and there are double sharps or flats, which raise or lower the indicated note by two semitones. Accidentals usually apply to all repetitions within the measure in which they appear, unless canceled by another accidental sign, or tied into the following measure. If a note has an accidental and the note is repeated in a different octave within the same measure the accidental is usually repeated, although this convention is far from universal.
The modern accidental signs derive from the two forms of the lower-case letter b used in Gregorian chant manuscripts to signify the two pitches of B, the only note that could be altered. The "round" b became the flat sign, while the "square" b diverged into the sharp and natural signs.
Sometimes the black keys on a musical keyboard are called "accidentals" (more usually sharps), and the white keys are called naturals.
In most cases, a sharp raises the pitch of a note one semitone while a flat lowers it one semitone. A natural is used to cancel the effect of a flat or sharp. This system of accidentals operates in conjunction with the key signature, whose effect continues throughout an entire piece, unless canceled by another key signature. An accidental can also be used to cancel a previous accidental or reinstate the flats or sharps of the key signature.
Accidentals apply to subsequent notes on the same staff position for the remainder of the measure where they occur, unless explicitly changed by another accidental. Once a barline is passed, the effect of the accidental ends, except when a note affected by an accidental is tied to the same note across a barline. Subsequent notes at the same staff position in the second or later bars are not affected by the accidental carried through with the tied note.
Under this system, the notes in the example above are:
Though this convention is still in use particularly in tonal music, it may be cumbersome in music that features frequent accidentals, as is often the case in atonal music. As a result, an alternative system of note-for-note accidentals has been adopted, with the aim of reducing the number of accidentals required to notate a bar. According to Kurt Stone, the system is as follows:
Because seven of the twelve notes of the chromatic equal-tempered scale are naturals (the "white notes", A; B; C; D; E; F; and G on a piano keyboard) this system can significantly reduce the number of naturals required in a notated passage.
Occasionally, an accidental may change the note by more than a semitone: for example, if a G♯ is followed in the same measure by a G♭, the flat sign on the latter note means it is two semitones lower than if no accidental were present. Thus, the effect of the accidental must be understood in relation to the "natural" meaning of the note's staff position.
In some atonal scores (particularly by composers of the Second Viennese School), an accidental is notated on every note, including natural notes and repeated pitches. This system was adopted for "the specific intellectual reason that a note with an accidental was not simply an inflected version of a natural note but a pitch of equal status."
Double accidentals raise or lower the pitch of a note by two semitones, an innovation developed as early as 1615. This applies to the written note, ignoring key signature. An F with a double sharp applied raises it a whole step so it is enharmonically equivalent to a G.
Usage varies on how to notate the situation in which a note with a double sharp (or flat) is followed in the same measure by a note with a single sharp (or flat). Some publications simply use the single accidental for the latter note, whereas others use a combination of a natural and a sharp (shown below), with the natural being understood to apply to only the second sharp.
The double accidental with respect to a specific key signature raises or lowers the notes containing a sharp or flat by a semitone. For example, when in the key of C♯ minor or E major, F, C, G, and D contain a sharp. Adding a double accidental (double sharp) to F in this case only raises F♯ by one further semitone, creating G natural. Conversely, adding a double sharp to any other note not sharped or flatted in the key signature raises the note by two semitones with respect to the chromatic scale. For example, in the aforementioned key signature, any note that is not F, C, G, and D is raised by two semitones instead of one, so an A double sharp raises the note A natural to the enharmonic equivalent of B natural.
In modern scores, a barline cancels an accidental, with the exception of tied notes. Courtesy accidentals, also called cautionary accidentals or reminder accidentals are used to remind the musician of the correct pitch if the same note occurs in the following measure. The rules for applying courtesy accidentals (sometimes enclosed in parentheses) vary among publishers, though in a few situations they are customary:
Publishers of free jazz music and some atonal music sometimes eschew all courtesy accidentals.
Composers of microtonal music have developed a number of notations for indicating the various pitches outside of standard notation. One such system for notating quarter tones, used by the Czech Alois Hába and other composers, is shown on the right.
In the 19th and early 20th centuries, when Turkish musicians switched from their traditional notation systems—which were not staff-based—to the European staff-based system, they refined the European accidental system so they could notate Turkish scales that use intervals smaller than a tempered semitone. There are several such systems, which vary as to how they divide the octave they presuppose or the graphical shape of the accidentals. The most widely used system (created by Rauf Yekta Bey) uses a system of four sharps (roughly +25 cents, +75 cents, +125 cents and +175 cents) and four flats (roughly −25 cents, −75 cents, −125 cents and −175 cents), none of which correspond to the tempered sharp and flat. They presuppose a Pythagorean division of the octave taking the Pythagorean comma (about an eighth of the tempered tone, actually closer to 24 cents, defined as the difference between seven octaves and 12 just-intonation fifths) as the basic interval. The Turkish systems have also been adopted by some Arab musicians.
Ben Johnston created a system of notation for pieces in just intonation where the unmarked C, F, and G major chords are just major chords (4:5:6) and accidentals create just tuning in other keys. Between 2000 and 2003, Wolfgang von Schweinitz and Marc Sabat developed the Extended Helmholtz-Ellis Just Intonation (JI) pitch notation, a modern adaptation and extension of the notation principles first used by Hermann von Helmholtz, Arthur von Oettingen, and Alexander John Ellis that some other musicians use for notating extended just intonation.
The three principal symbols indicating whether a note should be raised or lowered in pitch are derived from variations of the small letter b: the sharp (♯) and natural (♮) signs from the square "b quadratum", and the flat sign (♭) from the round "b rotundum" b.
The different kinds of B were eventually written differently, so as to distinguish them in music theory treatises and in notation. The flat sign ♭ derives from a round b that signified the soft hexachord, hexachordum molle, particularly the presence of B♭. The name of the flat sign in French is bémol from medieval French bé mol, which in modern French is bé mou ("soft b"). The natural sign ♮ and the sharp sign ♯ derive from variations of a square b that signified the hard hexachord, hexachordum durum, where the note in question is B♮. The name of the natural sign in French is bécarre from medieval French bé quarre, which in modern French is bé carré ("square b"). In German music notation, the letter B or b always designates B♭ while the letter H or h – a deformation of a square b – designates B♮.
In the early Middle Ages, a widespread musical tradition was based on the hexachord system defined by Guido of Arezzo. The basic system, called musica recta, had three overlapping hexachords. Change from one hexachord to another was possible, called a mutation. A major problem with the system was that mutation from one hexachord to another could introduce intervals like the tritone that musicians of the time considered undesirable. To avoid the dissonance, a practice called musica ficta arose from the late 12th century onward. This introduced modifications of the hexachord, so that "false" or "feigned" notes could be sung, partly to avoid dissonance. At first only B could be flattened, moving from the hexachordum durum (the hard hexachord) G–A–B–C–D–E where B is natural, to the hexachordum molle (the soft hexachord) F–G–A–B♭–C–D where it is flat. The note B is not present in the third hexachord hexachordum naturale (the natural hexachord) C–D–E–F–G–A.
Strictly speaking the medieval signs ♮ and ♭ indicated that the melody is progressing inside a (fictive) hexachord of which the signed note is the mi or the fa respectively. That means they refer to a group of notes around the marked note, rather than indicating that the note itself is necessarily an accidental. For example, when a semitone relationship is indicated between F and G, either by placing a mi-sign (♮) on F or a fa-sign (♭) on G, only the context can determine whether this means, in modern terms, F♯-G or F-G♭, or even F♭–G
As polyphony became more complex, notes other than B required alteration to avoid undesirable harmonic or melodic intervals (especially the augmented fourth, or tritone, that music theory writers referred to as diabolus in musica, i.e., "the devil in music"). Nowadays "ficta" is used loosely to describe any such un-notated accidentals. The implied alterations can have more than one solution, but sometimes the intended pitches can be found in lute tablatures where a fret is specified.
The convention of an accidental remaining in force through a measure developed only gradually over the 18th century. Before then, accidentals only applied to immediately repeated notes or short groups when the composer felt it was obvious that the accidental should continue. The older practice continued in use well into the 18th century by many composers, notably Johann Sebastian Bach. The newer convention did not achieve general currency until early in the 19th century.