Lesson 3 - Section 1
Pretend you are still a composer writing a new song and the new song is in the Key of D (2 sharps). In the melody line, you find that one of the notes that is normally played as a sharp needs to be played as a non-sharp. To do this, you would label it with a Natural note sign. Look at the image for this section. See the C on the staff? The Key Signature says to sharp all C‘s. However, see the little parallelogram to the left of the C note? This is a Natural sign.
Lesson 3 - Section 2