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victoria Williams Music Theory

Victoria Williams

LmusTCL BA Mus (Hons) MISM

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6. Key Signatures & Accidentals

Grade Two Music Theory Lesson 6: Key Signatures & Accidentals

Suitable for:  ABRSM Grade 2   Trinity Grade 2   GCSE   AP Music Theory Beginners 

Keys and Key Signatures

If a melody uses mostly the notes of the Bb major scale, we say that the music is "in the key of" Bb major.

We don't write out the flat symbols for the Bs and the Es every time they appear in the music - because there would probably be rather a lot of them! Instead, we use a key signature: at the beginning of each new line of music, we write a Bb and an Eb, to remind us that all the Bs and all the Es need to be flattened.

The key signature also tells us very quickly that the music is in Bb major, without having to count all the flats!

Here's a key signature of Bb major, with the note names marked under the melody:

B flat Key Signature - music theory 


Sometimes we need to add extra flats, sharps and naturals within a melody, even when we have already got a key signature. It might be because

  • the music changes key for a short time, or
  • just because they sound nice, or
  • because the music is in a minor key.

If we add sharps, flats and naturals inside the music itself, they are called "accidentals". Special rules apply to all accidentals.


Rules for Accidentals

Accidentals are always written on the left side of the note they affect. We write Correct sharp position - music theory and never Wrong sharp position - music theory.

Accidentals don't only affect the note they are next to. After an accidental has been written, every other note of the same position on the stave is also affected, but only until the next bar line. (Unlike key signatures, accidentals only affect the other notes at the same position on the stave.  Sharps and flats in key signatures affect all the notes with the same letter name, whatever their position on the stave.)

How Accidentals and barlines work in music theory

  • Note 1 is C natural
  • Note 2 is C sharp, because of the accidental
  • Note 3 is also C sharp, because it's in the same bar
  • Note 4 is C natural, because the sharp is "cancelled" (stopped) by the bar line

When a note is tied across a bar line, any accidental will also apply to the note in the next bar as well, even if there is no accidental.


  • Note 1 is Bb because of the key signature
  • Note 2 is B natural because of the accidental
  • Note 3 is also B natural, because it's tied to the previous B natural


Key Signatures WITH Accidentals

Now let's see what happens when we have both a key signature and accidentals together.

Here's a couple of bars of music in the key of F major, so the key signature has one flat, Bb:

Key signatures, accidentals and barlines - music theory

  • Note 1 is Bb, because of the key signature
  • Note 2 is B natural, because of the accidental
  • Note 3 is also B natural, because it's in the same bar as note 2
  • Note 4 is B flat, because the barline cancels (stops) the natural accidental



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