What is a key signature?

I'm trying to learn the flute and I bought a book to help me learn. In one of the first chapters it describes something about key signatures and the circle of fifths and I'm completely lost.

What is a key signature? What does it mean when a song is "centered" around a certain note?

What steps do I have to take if I play a song that's in the key of B flat versus a song that's in C? What is the difference between C major and C minor? (Sorry for all the questions, but I'm completely lost.)

I think I have a C flute (I'm not sure what that means or if that makes any difference.)

Lastly, do I need to have a full grasp on these concepts before moving on and playing?

• While I agree with the great answers below, I have to state that the music theory is not actually a rigorous theory but an enumeration of what goes good with what. Just like the multiplication table, you can either memorize it by studying extensively or you do it over and over and it just sits in. So my naive advice is that don't pay too much attention before you actually play a piece or two. You'll eventually see that some pieces always follow certain patterns and then you can read up why this is happening. But right now it will keep confusing you. – user1306 Jan 4 '13 at 2:11

Even though you are learning the flute, I think it is worth understanding the piano keyboard, and thinking about key signatures in terms of that. It's worth experimenting with a piano, or an electronic keyboard, or even web app like this one.

The white keys are the 'natural' notes A-G. The black keys are used to play sharps and flats. Notice that B flat is the same piano key as A sharp, and so on.

You've probably noticed there aren't black keys between all the white keys. There is no B#/Cb or E#/Fb key -- that's just one of those things; I suggest getting used to the basics before trying to understand why this is.

The first scale most people learn is C major. That uses just the white keys, with no sharps or flats.

You'll find you can play lots of simple Western melodies using just the white keys. Try Twinkle Twinkle Little Star - C,C,G,G,A,A,G F,F,E,E,D,D,C

That melody begins and ends on C; but more generally it 'feels' as if C is the 'home' note. Try ending on a different note -- somehow the tune feels unresolved. So the melody is 'centred' on C.

So this is Twinkle Twinkle in C major because:

• C feels like the home note
• It uses only the white keys, which is the C major's key signature

Now let's try playing Twinkle Twinkle in D. What happens if we just move all the notes up by one white key? D,D,A,A,B,B,A G,G,F,F,E,E,D

Actually it sounds great, until you hit the F. The F sounds wrong. You can tell why, by counting the black keys between notes. When we played that part of the melody in C, it stepped from F to E - that's one semitone because there is no black key between F and E. Now we're stepping from G to F there's a black key in between, making it two semitones.

To fix that, play an F# instead. D,D,A,A,B,B,A G,G,F#,F#,E,E,D

What we've just done is called transposing - moving a piece of music from one key to another. We've transposed Twinkle Twinkle from C major to D major.

Twinkle Twinkle doesn't contain the 7th note of the scale, but you can see that the step from A to B is two semitones, and the step from B to C is only one semitone. So to fix that, in D major we play C# instead.

So - to transpose something from C major to D major, move everything up by one note letter, play F# instead of F, and C# instead of C.

When writing music on a stave, it gets messy having to put sharp and flat signs every time a black note is played. Instead they are marked at the beginning of the piece, just after the clef. If we were writing down our version of Twinkle Twinkle in D major, we would write the clef, then a sharp sign each on the F and C lines, then the time signature.

It turns out that no two major keys have the same combination of sharps and flats, so a musician will look at those two sharp signs, and say "aha, D major" -- so that notation is called a key signature, because it is an easily recognisable indication of what key we are using.

But it's not quite that simple, because we also have minor keys. A minor has no sharps of flats, just like C major. The only way to differentiate is to recognise that the melody is centred on A. Likewise, B minor has the same sharps and flats as D major.

There are more scales too -- but that's a more advanced topic, to be tackled when you're comfortable with these basics.

All of this is exactly the same on your flute; it's just not as visually obvious as on a piano keyboard.

• Wow, the Twinkle Twinkle example really makes sense, thank you so much! This has really helped me out a ton! – Melanie Shebel Jan 4 '13 at 6:04
• "There is no B#/Cb or E#/Fb" Has something changed since I was taught music theory 45 years ago? – JimM Jul 16 '16 at 20:58
• Just to clarify, @slim said "So - to transpose something from C major to D major, move everything up by one note letter, play F# instead of F, and C# instead of C." IMHO, this is misleading advice. Transposing from C to D involves TWO (notes) steps, not one. The provided examples compared different scale degrees. – Kirk A Oct 29 '16 at 10:46
• @JimM - I noticed that too, but slim's intent is clear. Yes, there are such notes, and learning about them is important; but getting into that at this point would be confusing. I've proposed an edit to clarify this. – neilfein Jan 2 '17 at 6:15
• +1 because everyone should understand the piano keyboard. It does wonders for your understanding of theory. – General Nuisance Apr 23 '18 at 17:41

A key signature is a bunch of zero or more sharp or flat signs written at the beginning of each line of music (sometimes only the first line). It tells you which notes are to be raised or lowered by a semitone by default.

When you start from C and play a scale of "just the names", that is

C, D, E, F, G, A, B and C, without any sharps or flats

you get a C-major scale (I'm assuming the convention that B means B natural). What makes it a major scale is the interval pattern between the notes. In this case it is

C->D = whole tone, D->E = whole, E->F = half, F->G = whole, G->A = whole, A->B = whole, B->C = half

or in numbers:

1, 1, 1/2, 1, 1, 1, 1/2.

Now go up a fifth from C to G. If you now play this interval pattern starting from G, you have to raise F to F sharp. Nothing else changes. Thus the G major scale is:

G, A, B, C, D, E, F#, G.

So, if you want to play in the key of G major, i.e. over the G major scale, you always have to raise F to F# (= F sharp). In other words, you always have to put a sharp sign (#) before every F in the sheet music. This is not very efficient so the key signature was invented. You now just put one sharp sign on the topmost line (i.e. where an F would go) at the beginning of each line to indicate that every F should be played F#.

Let's go up another fifth from G to D. Now if you play the major scale pattern from D you have to raise both F and C. Thus, the key signature for D major has two sharps, one at F-position and one at C-position. Yet another fifth up is A, and there you have to raise F, C and G. Going up fifths you get E, B, and finally F#. The key of F# major has six sharp signs in its key signature and you seldom get more than that.

But there are still notes left. Let's now go a fifth below C to F. If you play an F major scale you have to lower B to B flat (= Bb). So, this time the key signature for F major has one flat sign (b) in it. If you go a fifth below that to Bb you'll see (or rather hear) that the Bb major scale has two lowered notes: B flat and E flat. Going down fifths you get Eb, Ab, Db, and finally Gb with six flat signs in the key of Gb major. But Gb is just another name for F# (in equal temperament at least) so the circle has closed. You'll notice that we have now gone through every 12 notes (one of them even twice) so there's nothing left.

This circle is the circle of fifths, and by following it you'll find out which key signature matches which key: if the key signature has x sharps in it, you go x fifths up from C, and the key for that signature is the major key starting from that note. If the signature instead has x flats, you just go x fifths down from C. Conversely, if you want to find out the signature for x major, just start from C and go up or down by fifths until you hit x. Count the fifths, note whether you went up or down, and you have the amount of sharps or flats in the signature for x major. To find out exactly which notes to put the sharps or flats on, either consult a picture with all the key signatures written on it, or notice that these also follow the circle of fifths: for example sharps go like: F#, C#, G#, ... (fifths up), and flats like: Bb, Eb, Ab, ... (fifths down).

But what about minor keys? Just as the C major scale can be played by playing just the names, so can the A minor scale: A, B, C, D, E, F, G, A. The interval pattern for minor is therefore:

1, 1/2, 1, 1, 1/2, 1, 1.

If you go up a fifth from A to E and play the E minor scale, you have to raise F to F# but nothing else. Thus the key signature for E minor is the same as the one for G major. This is actually true for every note: to get the key signature for x minor just go a minor third up from x and see what the key signature for that major scale is. Similarly if you have the key signature for x major, go down a minor third from x and you get the minor scale which fits this key signature. This is called the relative minor of x.

Since C is a minor third below Eb, the key signature for C minor is the same as the one for Eb major, i.e. it has three flats: Bb, Eb, and Ab. In the scope of these questions, this is the difference between C major (which has no flats or sharps) and C minor.

Having a C flute just means that when you play a "logical C" (the fingering for C, I guess, I'm not a flutist) then what you hear is also a C. This means that the note you play is the one you hear, in other words, the instrument doesn't transpose. If you'd have, say, a G flute, then you'd hear a G when you play C. Similarly for any note you'd play, you'd hear the note a fourth below that. In that case the flute would transpose a fourth. As far as I know, every "normal" flute is in C so you don't have to think about this any more.

Also in the scope of this question, the meaning of "being centered around a note x" is that the key signature is the one for either x major or x minor. More accurately it means that the x major or minor chord is the "most restful" chord of the piece, and others have more tension. However, this should really be a question of its own.