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In musical notation, an accidental is the use of a symbol such as a sharp or flat in the course of a piece, as opposed to in its key signature. This indicates that the note on the staff is altered from the pitch it normally represents. This reverts at the end of the measure.
The term "accidental" presumably refers to the older sense of the word "accidental" meaning "outside the norm", since the notes affected by them fall outside the scale of the current key.
All accidentals, regardless of the current key, modify their following notes as if they began in the key of C, as follows:
- A sharp raises the pitch by one semitone
- A double-sharp raises the pitch by two semitones
- A flat lowers the pitch by one semitone
- A double-flat lowers the pitch by two semitones
- A natural cancels the effect of any previous sharp or flat (including in the key signature). Any note following a natural mark will be part of the key of C major.
When canceling from a double-sharp to a single sharp, it is acceptable to just write a sharp sign, but better practice to write "natural, sharp" in succession.
Example
The top staff has a key signature is two flats (either B flat major or G minor)
- a B with no sign: thus a B-flat, on key
- a B-natural: the natural sign cancels the implicit flat on the key signature
- a B-double-flat, two semitones below B (enharmonic to A)
- an F-double-sharp, two semitones above F
- an F-natural, canceling the double-sharp
- another F-natural, without the need for a second accidental
- an F-sharp, one semitone above F
- another F-sharp.
The bottom staff gives the exact same pitches, technically called enharmonic spellings, written in the key of C.
Writing Accidentals
When an accidental note is tied across a barline, no additional accidental is needed, as it is implied by holding the note. The next occurrence of that note in the second bar will be in key unless given an accidental of its own.
Although a barline implicitly resets all lines and spaces to the last key signature, typically a courtesy accidental will be placed to remind performers in some of the following situations:
- The unaltered note in the bar which follows the altered note
- The same note as the altered note occurs in a different octave but is itself unaltered
- The altered note is tied across a barline, and followed by an unaltered note.
The rules for which accidentals to choose may vary according to the type of music: modal, diatonic or chromatic, and also whether the transcriber is aiming for strictness or clarity, for example C flat versus B in the key of D flat. Nonetheless, some general rules for choosing between flat or sharp accidentals include:
- When descending, use flats.
- When ascending, use sharps.
- Try to use the same kind of accidentals -- sharps or flats -- used by the key signature.
On a piano or other equally tempered instruments with fixed tuning, a sharp and a flat are the same distance from the natural note in either direction (so that C sharp is the same as D flat - they are enharmonically equivalent), while instruments with flexible tuning such as violins or cellos are often played so they more closely approximate just intonation, meaning sharps tend to be lower and flats higher (so that C sharp is slightly lower than D flat).
In the 19th and beginning 20th century when Turkish musicians switched from their traditional notation systems which were not staff based to the European staff based system, they created a refinement to the European accidental system in order to be able to notate Turkish scales which make use of intervals smaller than the tempered semitone. There are several such systems which vary as to the division of the octave they presupppose or merely the graphical shape of the accidentals. The most widely used system (created by Rauf Yekta Bey) uses a system of 4 sharps (roughly +25 cents, +75 cents, +125 cents and +175 cents) and 4 flats (roughly -25 cents, -75 cents, -125 cents and -175 cents) (these values are not precise enough but I don't have my notes with me; correct later) none of which correspond to the tempered sharp and flat. They presuppose a Pythagorean division of the octave taking the Pythagorean comma (about an 8th of the tempered tone, actually closer to 24 cents, defined as the difference between 7 octaves and 5 just-intonation 5ths) as the basic interval. The Turkish systems have also been adopted by some Arab musicians. From the 20th century, some European composers (such as the Czech Alois Hába and others, see Microtonal music) have used other accidentals, commonly half- and quarter-sharps and -flats. These are usually indicated by the use of altered traditional symbols, such as a backwards flat to indicate half-flat. 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 are used to create just tuning in other keys.
History of accidental notation
In the beginning of European music notation (4-line staff Gregorian chant scores) only B could be altered (i.e. applied an accidental to: it could be flattened, thus moving from hexachordum durum (i.e. hard hexachord: G-A-B-C-D-E) where it is natural, to hexachordum molle (i.e. soft hexachord: F-G-A-Bb-C-D) where it is flat; B is not present in the third hexachord hexachordum naturale (i.e. natural hexachord: C-D-E-F-G-A)). (To make this clearer one may have to explain the procedure of Guidonian mutations and generally Guidonian sol-fa).
This long use of B as the only altered note incidentally helps explain some notational peculiarities: the flat sign actually derives from a round B, to signify the B of the soft hexachord, i.e. B flat (hence the name of the flat sign in French "bémol" from medieval French "bé mol" — modern French "bé mou" — or "soft b") and originally meant only Bb; the natural sign derives from a square B, to signify the B of the hard hexachord, i.e. B natural (hence the name of the natural sign in French "bécarre" from medieval French "bé carre", earlier "bé quarre" — modern French "bé carré" — or "square b") and originally meant only B natural. In the same way, in the German notation the letter B only designates the B flat while the letter H, which is actually a deformation of a square B designates the B natural. (It might also have helped that H was the letter which followed G so that it completed the alphabetic series A, B, C, D, E, F, G, H, but that is not the reason Germans use H for B natural, it is at best an a posteriori justification; rather the real origin is that stated above, namely a deformation of a square B)
As polyphony developped other notes (than B) needed to be altered (either sharped or flattened, the first sharp in use was F#, then came the second flat Eb, then C#, etc.; by the 16th century Bb, Eb, Db, Ab, Gb and F#, C#, G#, D# and A# were all in use) in order to avoid undesirable harmonic intervals (especially the augmented 4th that theory writers called "diabolus in musica", i.e. "the devil in music").
(I don't recall what the origin of the sharp and double-sharp signs is, but it is completely different from that of the flat and natural which are older by a couple of centuries.)
However those accidentals were often not notated in vocal scores (but were always notated in tablatures). This notational practice of not marking implied accidentals, leaving them to be supplied by the performer instead, was called "musica ficta" (i.e. "feigned music").
In some of the autographs of the 17th and 18th one does not notice the incoherence of the modern accidental notation, in which the value of the altered note does not depend on the key signature contrary to the value of the unaltered note, which, among other things, makes sight transposition more awkward that it ought to be.
In such scores (e.g. autographs by J.S. Bach) the value of the altered note does in fact (as it ought to) depend on the key signature. For example a flat sign before B means Bb in the key of C major but Bbb (B double-flat) in the key of F major. Similarly a sharp sign before A means Ab in the key of C major but A natural in the key of Eb major. A natural sign before B means B natural in the key of C major but Bb in the key of F major and it needs to be used only if the B had been previously flattened or sharpened. (Generally speaking the natural is always used to restore the value of the note to that of the key signature, never to cancel an accidental in the key signature.)
Not only does this manner of notating accidentals avoid the awkwardness in sight transposition alluded to above, but it also almost never requires the use of double-flats and double-sharps. Also the natural sign is always used to restore the value of the note to that of the key signature, never to cancel an accidental in the key signature.
Modern editions of course always change this style of marking accidentals to the modern style (in the case of full scores, not necessarily in the case of a figured bass, see below) even if the original score is notated in the old manner. But this style can still be seen in some editions for the notation of figured bass chords even today (or at least quite recently, long after this style of notation had been abandoned for scores.)
(to be checked and expanded)
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