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Grade Two Music Theory Lesson 6: Key Signatures & Accidentals
Keys and Key Signatures
If a melody uses mostly the notes of the Bb major scale, in music theory 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:
Accidentals
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  and never  .
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 staff is also affected, but only until the next barline.
- 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 barline
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:
- 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
Now you are confident working with both key signatures and accidentals, we'll move on to the types of question you might get in Grade Two Music Theory which involve using them in the next lesson.
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