Here is another one of those Bach Chorales everyone has been talking about.
I am currently taking an online songwriting class. The latest assignment was to write a song in AABA format, using all of the chords in any one key, without modulating to another key.
As you can see (or hear), when a song just stays in the same key it gets monotonous very quickly.
One song that is interesting, despite not modulating, is Ravelle’s Bolero. It stays in the key of C for almost the entire song. Do you remember the Dudley Moore/Bo Derrick film that made this song famous?
The Write Stuff 