Joni Mitchell's "Help Me" from her classic 1974 album, Court and Spark, provides a wonderful example of how a key can be expanded through the use of parallel harmony. I'll define this term in a minute, but first this: One way of thinking of a key in music is as a house. And a house can be simple and easy to understand immediately, or it can be complex with many rooms and hallways and so on. But in both cases, for the house to be beautiful, it must be clearly only one house. In music, the chords built from only the notes of a key are called diatonic, and they have been used to harmonize all types of melodies in all types of music. They are crucial, but most musicians try to find ways to expand keys by bringing in sounds and chords from other keys. These chords (and notes) are called chromatic (due to their colourful quality, I suppose). Now back to parallel harmony. This term means that the structures themselves (rather than the individual notes) create the glue that holds the piece together. In "Help Me", it is the structure known as the major seven chord that is used to accomplish this.

Now the diatonic chords in the key of G are the following: G major seven, A minor seven, B minor seven, C major seven, D seven, E minor seven, and F# minor seven (flat five). Notice that there are only two major seven chords. OK, here are the chords in "Help Me" (also in the key of G): G major seven, F major seven, C major seven, Bb major seven, D major seven, A major and E minor. So there are five different major seven chords used, and each one takes us into another chromatic room in this glittering mansion of a song.