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What a key signature is
A key signature is the group of sharps or flats written at the start of every staff in a piece of music. It tells players which notes are sharp or flat throughout the piece, unless a note says otherwise. Instead of writing a sharp sign before every F, the composer writes one sharp in the key signature, and every F in the piece is sharp.
Key signatures also tell you the key of the piece, or at least narrow it down. Three sharps means A major or F sharp minor. Two flats means B flat major or G minor. Every signature belongs to one major key and one minor key, called relatives, because they share the same notes.
How this finder works
There are two ways to search. First, pick sharps or flats and a count from zero to seven, and the tool gives the major key, the relative minor, and the accidentals in order. Second, tick the notes in your melody, and the tool lists every major and minor key whose scale contains all of those notes.
The order of sharps never changes: F sharp, C sharp, G sharp, D sharp, A sharp, E sharp, B sharp. The order of flats never changes either: B flat, E flat, A flat, D flat, G flat, C flat, F flat. Learn one order and you know the other, because the flat order is the sharp order backwards.
The circle of fifths
Key signatures follow the circle of fifths. Start at C major, with no sharps or flats. Move up a fifth to G major, one sharp. Up another fifth to D major, two sharps. Keep going and the sharps pile up until C sharp major, with seven sharps. Go the other way, down a fifth each time, and the flats pile up instead.
The circle is worth memorising because it organises all of Western harmony. Keys next to each other on the circle sound close. Keys opposite each other sound far apart. Modulating, changing key inside a piece, usually moves to a nearby key on the circle.
Major and relative minor
Every major key has a relative minor that shares its key signature. The relative minor starts a minor third below the major tonic, or six semitones below, which is the same note. A major’s relative minor is F sharp minor. B flat major’s relative minor is G minor.
The two keys share notes but feel different, because the centre of gravity is different. The same set of notes can sound bright or dark depending on which note feels like home. That is why a piece in three sharps could be cheerful A major or brooding F sharp minor, and only the music itself tells you which.
Using the note search
The note search is useful when you are working out a song by ear. You know some of the notes, but not the key. Tick the notes you are sure about, and the tool narrows the keys. Tick more notes and the list shrinks. If the list empties, one of your notes is wrong, or the song changes key, or it uses notes outside the key, which happens often in real music.
Be careful with chromatic notes. A song in C major can still contain an F sharp as a passing note. The note search assumes every ticked note belongs to the scale, so tick only the notes you hear often and confidently.
Reading a real key signature
On a real staff, sharps and flats sit on specific lines and spaces, in a fixed pattern, and the pattern differs between treble and bass clef. The count is what matters for finding the key: count the symbols, and the tool does the rest. With practice, you will recognise common signatures at a glance: one sharp is G major, two flats is B flat major, and no symbols is C major or A minor.
Use this finder while you practise reading music. It is free, runs in your browser, and needs no sign-up.