Key Signature Identifier

Runs entirely in your browser — no audio or data ever leaves your device (see methodology).

How It Works

People arrive at key signatures asking one of two questions, and a printed table can only be organised around one of them at a time. Either you are looking at a stave, you have counted three sharps, and you want to know what key that is — or you know the piece is in A flat major and you want to know what its signature looks like so you can write it. This tool runs in both directions off the same table, because the underlying fact is a single one viewed from either end. The count-to-key direction is the one search engines see most often, phrased almost exactly as it is asked out loud: one sharp is what key, three sharps key signature, seven sharps key signature. Press the number of accidentals you can see and the answer appears immediately, in both its major and minor forms, along with the specific reading rule that produces it. The rules themselves are worth knowing rather than merely being given the answer to, because they work at sight without any lookup: for sharps, the last sharp written is a semitone below the major tonic, so a signature ending in F sharp is G major and one ending in G sharp is A major. For flats, the second-to-last flat names the major key directly, so a signature running B flat, E flat, A flat is E flat major. There is exactly one exception in each direction and the tool names it where it applies — an empty signature has nothing to count, and a single flat has no second-to-last flat to read, so F major has to be memorised outright. The key-to-signature direction draws the answer rather than describing it. A signature is a position, not a list: the accidentals go on specific lines and spaces, in a fixed order that never varies, and they sit in different places in the treble and bass clefs. Being told that D major has F sharp and C sharp is half an answer; seeing the two sharps on the top line and the third space, in that order, is the other half. The stave here can be switched between treble and bass clef precisely because the placements differ, and a musician reading a bass part needs the bass positions rather than a mental transposition of the treble ones. Fifteen signatures are covered rather than twelve, and the extra three are the part of this topic most reference material glosses over. Three pitch levels can be written two ways, and both spellings are in genuine use: B major with five sharps is the same sound as C flat major with seven flats; F sharp major with six sharps is the same sound as G flat major with six flats; C sharp major with seven sharps is the same sound as D flat major with five flats. Where one spelling clearly wins on accidental count the tool says so and offers a single click to switch to it, because a musician who has just worked out that a passage is in C sharp major almost always wants to know that everyone else would have written D flat. The six-sharp and six-flat pair is different: it is an exact tie, neither spelling is simpler, and the tool says that plainly instead of pretending there is a right answer. Every result also shows what the signature is actually for. The two scales it produces — the major and its relative minor — are listed with their correct letter spellings and can be played back, so the abstract count turns into a sound. Underneath them are the seven diatonic chords of the major key, each linking through to the chord builder set to that exact chord, because the practical reason to identify a key is usually that you are about to play something in it. Correct spelling matters here in a way that a pitch-only tool cannot deliver: B flat major is spelled B flat, C, D, E flat, F, G, A, using each letter exactly once, and calling those notes A sharp and D sharp would put two accidentals on one staff line and none on another, which is not what a stave draws. Everything runs in your browser from a table that ships with the page. Nothing is fetched, nothing is sent anywhere, and the tool works with no connection at all once the page has loaded. Every state is written into the address bar, so a particular signature is a link you can send to a student or paste into a forum answer, and each of the fifteen also has its own permanent page with the same tool locked to it, written up in more depth — including which instruments are pitched in that key and why some signatures that look hard on paper are the easiest ones to actually play.

FAQ

How do I work out the key from the sharps without a tool?

Look at the last sharp in the signature and go up one semitone. A signature ending in C sharp is D major; one ending in A sharp is B major. For the relative minor, count three semitones down from that major tonic.

Does the flats rule work for one flat?

No, and that is the only place it fails. The rule reads the second-to-last flat, and a signature with one flat has no second-to-last flat. One flat is F major, and it has to be learned rather than derived.

Why are there fifteen key signatures rather than twelve?

Because three pitch levels are written two ways and both spellings are in real use — B and C flat, F sharp and G flat, C sharp and D flat. Reducing them to twelve would hide the enharmonic cases, which are the part people most often need explained.

Does the key signature tell me whether a piece is major or minor?

No. Each signature serves one major key and one minor key equally. What distinguishes them is the music: which chord it resolves to, and whether accidentals keep appearing that raise the minor key's seventh degree.