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The problem it solves
Between some natural notes (the white keys) there's a whole tone of distance. To reach the sound that sits right in between —and to be able to write it— you need a way to raise or lower a note by a half step.
Detailed theory
Key idea
A sharp raises the note a half step (a semitone); a flat lowers it a half step.
A natural undoes any alteration and returns the note to its natural sound.
That half step is the semitone, the smallest distance in Western tonal music; named as an interval it is a minor 2nd (for example E→F or C→C#).
Understand it
On the keyboard, between most white keys there is a black key: it is the sound that sits halfway, a semitone away. Accidentals are the symbols that let you write and play these in-between sounds.
The sharp (#) raises the note a half step: F# is the key immediately to the right of F. The flat (b) lowers it a half step: Bb is the key immediately to the left of B. The natural (♮) cancels the alteration and returns the note to its natural sound.
The half step that an alteration moves is the semitone: the smallest possible distance between two notes in Western tonal music. When this step is named as an interval, it is a minor 2nd; for example, from E to F or from C to C# there is exactly one semitone, that is, a minor 2nd.
An accidental is always written just to the left of the note head and, when it is accidental, affects every note of the same name within that measure until a natural or a new barline undoes it.
Think of an elevator between two floors: the sharp is going up one floor, the flat is going down one, and the natural is returning to the floor you started on. The person's name (the note) doesn't change; only the floor they are on.
Staff & keyboard
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Not every semitone uses a black key: between E and F (and between B and C) there is already a natural semitone, with no key in between.
How to recognise it
How it's written
Look for the symbol #, b or ♮ attached to the left of the note head: it tells you whether to play it a half step up, a half step down, or natural. When it is an accidental, it holds for the rest of the measure.
How it feels
Play a natural note and then the same one sharpened: you'll hear a small step up, the shortest possible jump between two notes. The flat makes the same step, but downward.
Common mistake
Confusing the direction: a sharp always goes up and a flat always goes down, regardless of whether the resulting key is black or white.
Forgetting that an accidental keeps holding until the end of the measure, not just for the note where it appears.
Try it
On the keyboard, play F and then F#: it's the shortest step to the right, just a half step.
Play B, then Bb (the black key to the left) and finally return to B: you have raised, lowered and reset the same note.
On the instrument
Staff & keyboard
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The same note, G, lowered a half step (Gb), natural (G) and raised a half step (G#). The flat goes to the left, the sharp to the right; a natural would leave it natural again.
Where it's used
- Modifying the pitch
- Raising or lowering a note a half step to adapt a melody or create a new harmonic quality.
- Reading scores
- Recognising the sharp, flat and natural symbols placed immediately to the left of the note head.
Examples
Staff & keyboard
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Three half steps in a row: F, F# and G. Each jump is the smallest possible, a semitone. Listen to it rise.
Staff & keyboard
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An F sharp and, right after, the natural F: here the natural undoes the alteration and the note returns to its original sound.
Exercises
Recognise notes with accidentals
Read notes with sharps, flats and naturals and identify them.
Complete 10 attempts · 70% accuracy to pass
Mini test
Check that you've got it.
0/7 answeredQuestion 1/7
What does a sharp (#) do to a note?