On a staff of music, a group of accidentals between the clef symbol and time signature indicate the diatonic key of a song.
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Example: The key of G has one sharp ( G, A, B, C, D, E, and F♯) so a # is indicated on the top line of the treble staff and the fourth line of the bass staff.
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The key of C has no sharps or flats. In this case, the lack of accidentals displayed on the staff indicates the song is in C.