Understanding Key Signatures

What is a key signature?

A key signature is the group of sharps/flats written at the beginning of the piece.

Why do we use key signatures?

They keep the page clean by stating the sharps/flats once, up front.

How many sharps or flats does C major have in its key signature?

C major uses only white keys, so its key signature is empty. G major has one sharp (F♯); F major has one flat (B♭).