What is the purpose of the key signature?

Key signature, in musical notation, the arrangement of sharp or flat signs on particular lines and spaces of a musical staff to indicate that the corresponding notes, in every octave, are to be consistently raised (by sharps) or lowered (by flats) from their natural pitches.

1. Is key signature important?
2. What notes are affected by the key signature?
3. What is the purpose of the top number in the key signature?

Is key signature important?

The key signature appears right after the clef symbol on the staff. In common notation, 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 found on each line and space of the staff.

What notes are affected by the key signature?

The sharps and flats that appear in the key signature affect every note of that name in the entire piece, no matter what octave the note appears in.

What is the purpose of the top number in the key signature?

The top number of the time signature tells how many beats are in each measure, and the bottom number tells which note will represent one beat.