To understand what an accidental is, you must first understand what a key signature is.
That is answered at: What is a key signature? ..
but briefly, a key signature is a set of markings telling you which notes to always play as sharps or flats.
For example, the key signature for F major consists of a ♭ in the B position, meaning "Whenever a B appears in this score, play B♭".
An accidental points out a note which is an exception to the key signature.
It is a note with a ♯, ♮, ♭ next to it, to say "although the key signature says otherwise, for this bar only, play it like this".
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Accidentals are notes which don't usually occur in the key the piece of music is in.
For instance, if you're playing a piece in C major and there's a B flat, then this is an accidental note since B flat isn't in the scale of C major.
Accidentals are easy to spot in notation since they'll always have a sharp ♯, natural ♮, or flat ♭ sign in front of them.
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In the key of C, the "accidental" notes are the black keys on your piano.
In any other key, the "accidental" notes are whatever notes that you need to play in the piece which are not in the key itself , and thus not in the key signature.
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Don't forget also the (x) and the (bb).
Double sharps and double flats occasionally need to be used, to sharpen an already sharp (as in the written key signature) note, and vice versa.
E.g.
in Db, there is already a Bb in the key sig.
If one wanted to write a chord of Gbm; the third, normally a Bb, would need to be flattened again.
Thus it is written as Bbb or B double flat.
Yes I know lots of guitarists in particular would call it an A !!!
But technically it must be called a B note of some sort - hence Bbb.
These, I suppose, are actually the only accidentals that never get used as anything else.
Also, don't forget that after the next bar line following an accidental, all accidentals from the previous bar are cancelled.
It really bugs me that so many pieces of music put the cancelling natural, for example, in the next bar.
Totally pointless, yet seen in lots of piano exam pieces, especially at lower grades.
All it does is add extra superfluous marks to distract the player.
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The twelve tones in any chromatic scale are divided into two groups: Solfeggio and Accidental In the Key of C,do-re-me-fa-so-la-te (called solfeggio) are played on the white keys,where as di-ri-fi-si and li (called accidentals) are played on the black keys
The entire tonal chromatic scale is: do-di-re-ri-me-fa-fi-so-si-la-li-te
In the Key of C the accidentals are C# - D# - F# - G# - A# Solfeggio
form the roots of "Natural Chords" represented by Roman Numerals I-ii-iii-IV-V-vi-vii Accidentals
form the roots of the "Flatted Chords" flatted II, flatted III, flatted V, flatted VI and flatted VII (all played as major triads as the roman numerals are all capitals)
Thus all 15 Chromatic Scales have the same tonal labels, which makes transposing from the Key of A (easiest for guitarist) to the Key of C (easiest for the pianist) extremely easy.
Instead of thinking A is transposed to C, think: A is a 1 in the key of A and C is a 1 in the key of C.
That is why learning tonal scales and their corresponding roman numeral labels is a must for writing and transposing.
I'm a self-taught song writer and have spent many years searching through music theory books and talking to professionals.
I have talked to many piano teachers who have had no clue that the accidentals have tonal names, nor do they understand the need for Roman Numeral designations.
"What's the Circle of Fifths?" is a favorite common response I get from piano teachers.
I guess if you only want to read sheet music, there is no need for music theory, but it has helped me fully understand what I truly love: music!
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Can any one please explain what is an accidental note? Do they have any rules to play accidental notes in a scale? I only have basic knowledge of keyboards.
What are Accidental Notes?
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<terminology><accidentals>
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An accidental points out a note which is an exception to the key signature.  In the key of C, the "accidental" notes are the  black keys on your piano.    Accidental notes are notes that don't appear in the key that the piece of music is in. For example, in the key of C, the black keys on a piano are all accidental notes.
Accidental notes are notes which don't usually occur in the key of music that the piece is in. In the key of C, for example, the black keys on the piano are all accidental notes.