harmony                                                                                                                                                                                                      interval
KEY SIGNATURE

The key signature comes right after the clef symbol  and before the time signature at the beginning of the stave. It may have either some sharp symbols on particular lines or spaces, or some flat symbols. If there are no flats or sharps listed after the clef symbol,
 then the key signature is "all notes are natural".
The symbols that appear on the key signature depend on the scale which the song was composed.

sharps                 flat
    

  Clef and key signature are the only symbols that normally appear on every staff.
They appear so
often because they are such important symbols; they tell you what note is on each line and space of the staff. The clef tells you the letter name of the note (A, B, C, etc.), and the key signature tells you whether the note is sharp, flat or natural.
s f