The Lydian is called sub dominant because it lies just under the Dominant, the second most important mode after the Ionian, the Tonic.
Today, in song writing and jazz improvisation, we tend to think of the Lydian as the ‘go to’ mode after you’ve made your opening music statement. This doesn’t have to be so. In this set of books you’ll learn how to ‘go to’ any mode.
Here the focus is on the Lydian and how to make the best use of the information using the Lydian to develop ideas for the improvised line.