When a song consists of notes from a particular scale, it is said to be written in the key which has the same name as that scale. For example, if a song contains notes from the C major scale, it is said to be in the key of C major. The following song is written in the key of C major.
There are literally thousands of melodies which can be played using only the notes of the C major scale. It is extremely important to know all of the notes of this scale well and to be able to change between the notes easily. The following examples are designed to help you develop this skill.