What a key signature is
A key signature is the group of sharps or flats printed between the clef and the time signature at the start of every line. It is a standing instruction: every note of those letter names, in every octave and on every staff, is played sharp or flat for the whole piece, or until a new key signature appears. One sharp on the F line means every F on the page is F sharp.
- Sharps always appear in the order F, C, G, D, A, E, B; flats always appear in the reverse order B, E, A, D, G, C, F.
- To name a sharp key, take the last sharp and go up one semitone: one sharp is G major, two is D major, three is A major.
- To name a flat key, read the second-to-last flat: two flats is B flat major, three is E flat major; one flat is F major.
- A key signature lasts the whole piece; an accidental lasts to the end of its bar. Forgetting a key-signature sharp is the commonest wrong note in beginner playing.
The sharp or flat sign is drawn on one particular line or space, but it is not about that line. A sharp on the top-line F of the treble staff means every F: the F in the first space, the F on a ledger line, the F in the bass clef. Readers who take the sign literally sharpen one octave and forget the rest. The sign is a label for a letter name, placed where it is easiest to see.
The order of sharps and flats
Sharps are always added in the same order, F, C, G, D, A, E, B, and a key signature with three sharps always has the first three of that list, F sharp, C sharp and G sharp, never some other three. Flats are added in the reverse order, B, E, A, D, G, C, F. So three flats is always B flat, E flat and A flat. Each new sharp is a fifth above the last; each new flat is a fifth below.
The mnemonics: Father Charles Goes Down And Ends Battle for sharps, and backwards, Battle Ends And Down Goes Charles’ Father, for flats. Once you know the order you never need to read each sign; you count them. Four sharps is F, C, G, D sharp. Five flats is B, E, A, D, G flat. The signs on the staff are also always drawn in this order, left to right, so the last one, the rightmost, is the one the next section depends on.
Naming the key in one look
For sharp keys, find the rightmost sharp and go up one semitone: that is the major key. One sharp, F sharp, goes up to G: G major. Two sharps end on C sharp, up to D: D major. Three end on G sharp: A major. Four, D sharp: E major. Five, A sharp: B major. The last sharp is always the seventh note of the scale, the leading note, which is why the key is one step above it.
For flat keys, the second-to-last flat is the key. Two flats, B flat and E flat: the second-to-last is B flat, so B flat major. Three flats, B, E, A: E flat major. Four flats, B, E, A, D: A flat major. Five: D flat major. One flat has no second-to-last, so learn it: F major. And no sharps or flats at all is C major. That is two rules and two exceptions, and it covers every key signature you will meet.
Relative minors
Every key signature belongs to two keys: a major key and its relative minor, three semitones (a minor third) below it. One sharp is G major or E minor. Two sharps is D major or B minor. Three flats is E flat major or C minor. No sharps or flats is C major or A minor. The signature cannot tell you which; the music can. Look at the first bass note and the final chord: a piece in E minor starts and ends on E, and usually has D sharps written as accidentals, because the minor key raises its seventh note and the key signature does not include that.
Three semitones down is easy to count on a keyboard: from G, down to F sharp, F, E, so E minor. From E flat: D, D flat, C, so C minor. A table of the common ones: C major and A minor (none), G and E minor (one sharp), D and B minor (two sharps), A and F sharp minor (three sharps), E and C sharp minor (four sharps), F and D minor (one flat), B flat and G minor (two flats), E flat and C minor (three flats), A flat and F minor (four flats).
Key signature versus accidental
An accidental is a sharp, flat or natural written directly in front of a note inside a bar. It changes that note, and any repeat of the same note at the same pitch, until the end of that bar only. The next barline cancels it and the key signature is back in force. A natural sign is the accidental that cancels a key-signature sharp or flat for the rest of the bar. A tie carries an accidental across a barline for the tied note, but not for any new note after it.
Three things trip readers here. First, an accidental applies to the same pitch only: a G sharp in the treble staff does not sharpen the G an octave lower in the same bar (strictly; many engravers add a courtesy sharp there anyway). Second, a courtesy accidental in brackets is a reminder, not a change. Third, when a piece changes key signature mid-way, natural signs are printed to cancel the old sharps or flats, and the new signature then applies; a key change is the only thing that ends a key signature early.
A worked example
Picture a violin piece with two sharps after the clef, F sharp on the top line and C sharp in the third space. The last sharp is C sharp; one semitone up is D; the piece is in D major, or B minor. The first note is a D in the space just below the staff and the last chord of the piece is a D, so D major. Now read bar 4. The third note sits in the first space of the treble staff, F, with no sign in front of it. It is F sharp, because the key signature says so, even though nothing is written beside the note.
Bar 6 has a natural sign in front of a C in the third space. That C, and the C that repeats two notes later in the same bar at the same pitch, are C natural. The barline into bar 7 cancels the natural, and the C on the first beat of bar 7 is C sharp again. Bar 9 has a G sharp as an accidental: the piece is leaning toward A major for a moment, the sharp lasts the bar, and bar 10’s G is plain G.
The forgotten F sharp and other misreads
The forgotten F sharp is the classic. A player in G major reads every note correctly and plays every F as F natural, because nothing beside the note said otherwise. The piece sounds slightly wrong and the player cannot find the wrong note, because the wrong note is on a line that looks right. The fix is a habit: before the first note, say the key signature aloud, “one sharp, F sharp, G major”, and for the first few readings circle every F on the page in pencil.
The second misread is the octave trap described above: sharpening the F on the top line, where the sign is, and not the F in the first space. The key signature is a letter name, not a position.
The third is carrying an accidental into the next bar. A natural in bar 6 does not make bar 7 natural. Conversely, a sharp inside a bar does apply to the second time the same note appears in that bar, so the second G sharp in bar 9 is also sharp, even with no sign in front of it.
A two-minute drill
- Write the two orders, thirty secondsF C G D A E B for sharps, B E A D G C F for flats, from memory. Check them against the mnemonics.
- Name eight keys, one minuteOne, two, three and four sharps: G, D, A, E major. One, two, three and four flats: F, B flat, E flat, A flat major. Say each with its relative minor: E, B, F sharp, C sharp; D, G, C, F minor.
- Read one real signature, thirty secondsOpen any piece, count the sharps or flats, name the key, and say aloud which letters are altered. Then play the first bar with every altered note in place, or photograph the bar and hear it (below).
Hear the key
A key signature is easy to read and easy to forget halfway down the page, and the only reliable check is your ear against a correct performance. Sheetly is built for one job: turning the printed music you already own into something you can hear, ask about and practise, on an iPhone or iPad. Photograph the page, press play on bar 4, and the F in the first space sounds as F sharp, because the scan read the key signature too. Play your own bar 4 against it and the forgotten sharp is the note that clashes. Photograph it, hear it, understand it, practise it.
Read first, then check. Name the key, play the bar, then press play. At half speed a fast passage slows to the point where you can hear which notes are raised, and looping the bar where the accidental lives lets you hear the natural come and go at the barline.
Hear the key you just named.
Photograph the page, press play on one bar at any tempo, loop it, and ask the coach whether that F is sharp. Free to download on iPhone and iPad.
Free to download · iPhone and iPad · Android on the way
Ask about your bar
Sheetly is the only iPhone and iPad app we know of where the AI coach has read the actual notes of the page you photographed, so a question about bar 12 gets an answer about your bar 12. For key signatures that means you can ask what key the piece is in, or whether a particular note in a particular bar is sharp, and get an answer that has seen both the signature and the accidentals.
Ask Sheetly: a coach that has read your score
Not a chatbot bolted onto a music app. The coach has the actual notes of the page you just photographed, so you can ask about your bar 12 and get an answer about what is written in it.
We know of no other sheet-music scanner that does this. Every other tool hands you a transcription and stops.
Free to download · iPhone and iPadQuestions
What is the order of sharps in a key signature?
The order of sharps in a key signature is always F, C, G, D, A, E, B: Father Charles Goes Down And Ends Battle. A key signature with three sharps therefore always has F sharp, C sharp and G sharp. The order of flats is the reverse, B, E, A, D, G, C, F: Battle Ends And Down Goes Charles’ Father.
How do I tell what key a piece is in from the key signature?
For sharps, take the last sharp and go up one semitone: one sharp (F sharp) is G major, two sharps (last is C sharp) is D major, three sharps (last is G sharp) is A major. For flats, the second-to-last flat names the key: two flats (B flat, E flat) is B flat major, three flats is E flat major. One flat is F major and no sharps or flats is C major; those two you memorise. Each signature is also shared by a relative minor three semitones below the major, so check the first and last bass notes to tell which.
What key has two sharps?
Two sharps, F sharp and C sharp, is the key signature of D major and of its relative minor, B minor. The last sharp is C sharp, one semitone below D. Every F and every C in the piece, in every octave, is played sharp unless a natural sign cancels it for the rest of that bar.
What key signature has three flats?
Three flats, B flat, E flat and A flat, is the key signature of E flat major and of its relative minor, C minor. The second-to-last flat is E flat, which names the major key. Every B, E and A in the piece is played flat in every octave unless a natural sign cancels it for the rest of that bar.
Does a key signature apply to all octaves?
Yes. A key signature applies to every note of that letter name in every octave, on both staves, for the whole piece or until a new key signature is printed. The sharp sign is drawn on one line or space only, the top-line F in the treble clef for example, but it means every F on the page, including those on ledger lines and in the bass clef.
What is the difference between a key signature and an accidental?
A key signature is the sharps or flats printed after the clef at the start of every line; it applies to every note of those names, in every octave, for the whole piece. An accidental is a sharp, flat or natural written in front of one note inside a bar; it applies to that note and any repeat of it at the same pitch until the end of that bar only, after which the key signature is back in force. A tie carries an accidental across the barline for the tied note only.