Knowing how to read the key signature to determine what key to play is critical to reading music, but it isn't difficult. Count the number of sharps or flats in the key signature, and then you can use the circle of fifths (or the following table) to determine which major key to play in.
| If the Key Signature Has | The Music Is in the Key of |
|---|---|
| 1 sharp | G |
| 1 flat | F |
| 2 sharps | D |
| 2 flats | B flat |
| 3 sharps | A |
| 3 flats | E flat |
| 4 sharps | E |
| 4 flats | A flat |
| 5 flats, 7 sharps | C sharp/D flat |
| 6 flats, 6 sharps | F sharp/G flat |
| 7 flats, 5 sharps | B/C flat |
At the top you have the key of C major, which has no sharps or flats in its key signature. Each stop on the Circle as you go down clockwise from C is a key with one more sharp than the previous key. Each stop as you go down counter-clockwise from C is a key with one more flat than the previous key.
Sharps and flats are always added in a specific order that is linked to the circle of fifths. Flats are always added in this order: B, E, A, D, G, C, F. Sharps are added in the reverse order: F, C, G, D, A, E, B. Because of the regularity of sharps and flats and of the circle of fiths, if you don't have the circle of fifths memorized and don't have a picture of it handy, there are two simple tricks — one for sharps and one for flats — that you can use to figure out which major key to play in:
Sharps: Sharps always appear in the same order in a key signature. To figure out which major key you're in, look at the last sharp in the key signature. If it's on a line, go up to the next step; if it's a space, go up to the next line. That's what key you're in. For example, if you have four sharps — F#, C#, G#, and D# — look to the last sharp — D#. If you go up to the next note, you find what key you're in: E major.
Flats: Flats are even easier to figure out. Like sharps, they always appear in the same order. The next-to-last flat in the key signature is the name of the key you're playing in. For example, if you have five flats — Bb, Eb, Ab, Db, Gb — you just look at the next-to-last flat: you're in Db major. You will, though, just have to remember that if the key signature has only one flat (Bb), you're in F major.