The key of a song is based on the tonic.
You can tell what key a song is in by the number of sharps and flats on the staff.
| Sharps | Flats | ||
|---|---|---|---|
| Key | Number of # | Key | Number of ♭ |
| C | 0 | C | 0 |
| G | 1 | F | 1 |
| D | 2 | B♭ | 2 |
| A | 3 | E♭ | 3 |
| E | 4 | A♭ | 4 |
| B | 5 | D♭ | 5 |
| F# | 6 | G♭ | 6 |
| C# | 7 | C♭ | 7 |
Circle of Fifths
From here
