I am writing an tool to create music on a mobile device. I am going to allow the user to create simple melody and bass lines. I would like the user to be able to change key after they enter their melodies. As such I will store all musical phrases as degrees of the scale as opposed to storing the note number. This will mean that if I have a phrase
I, IV, V
I will easily be able to transpose it between keys.
My question is what scales should I allow the users to use? What is the essential set and what are others that would allow for interesting musical expression (that might be included in version 2 :)