Key signatures

The key signature in music is represented by one or many flats (b) or sharps (#), so-called accidentals (there are no accidentals shown for C Major since it has no flats or sharps). When using piano sheets, you will get the statement of the actual key at the beginning of a written music piece in which you can see symbols for flats or sharps near the staff, this is the key signature.

Let’s look at some examples:
G key symbol
This signifies the key of G. The F position on the sheet is marked with a sharp symbol and this is because the notes in the G Major scale are G, A, B, C, D, E, F#. But this is only logical if you are familiar with note reading.
F key symbol
This signifies the key of F. The flat symbol (b) is placed on the B line in the grand staff. The notes in the F Major scale are F, G, A, Bb, C, D, E.

In both cases the symbols # and b mean that you play on a black key instead of a white. The sharp (#) means the black key is to the right of the white key and the flat (b) means the black key is to the left of the white key. F# is, for example, the black key to the right of F and Bb is the black key to the left of B (see also Scale theory – Fundamentals).

In one case – the key of C (C, D, E, F, G, A, B) – there are neither sharps nor flats indicated:
C key symbol
What does “in the key of C mean”? It means that the music is played in the C Major Scale primarily, and that the tune normally starts and (especially) ends on a C note. Although, it is not necessary notes from the C major scale throughout the whole tune; other notes can be involved as well as key changes (the key change for a while and when return to the original key).

There are no specific symbols for Minor keys. A key signature doesn't have to indicate that the piece is played in Major, it can also be played in Minor. This is because Major keys have a relative Minor key (learn more about what distinguish keys from scales). So, if there is a song in A Minor, for example, it is indicated by a C key signature.

List of all keys with accidentals

A complete list of keys with flats or sharps:

C — nor flats or sharps.
G — one sharp.
D — two sharps.
A — three sharps.
E — four sharps.
B — five sharps.
F# — six sharps.
C# — seven sharps.
F — one flat.
Bb — two flats.
Eb — three flats.
Ab — four flats.
Db — five flats.
Gb — six flats.
G# — because of a double sharp it is normally not used.
D# — because of two double sharps it is normally not used.
A# — because of three double sharps it is normally not used.

A good thing to know about is that when keys are indicated, the sharps and flats are always in the same order:
Sharps — F#, C#, G#, D#, A#, E#, B#
Flats — Bb, Eb, Ab, Db, Gb, Cb, Fb

The F sharp key (the picture below) is indicated with six sharps in the following order, from left to right: F#, C#, G#, D#, A#, E#.
F sharp key symbol
See also Famous songs listed by key.