When a piece is not in the key of C Major or A Minor, it requires the use of regular accidentals. To avoid having to keep writing those accidentals, we can place them at the beginning of the piece using what is known as a key signature.
For instance, a piece in the key of D Major makes regular use of the notes of F-sharp and C-sharp, so the key signature uses those accidentals. When this key signature is present, all F and C notes are automatically raised and become sharp notes, unless the symbol of the natural accidental precedes them
Following we have a fragment from Beethoven’s famous Ode to Joy, in the key of D Major. Note how all the F and C notes are played sharp:
For more information see:
Translated by Dan Román, revised by Sue Talley.