Turn back to page 23 and the diagram for the diatonic scale, the basic scale of Western music. This diagram, of course, shows only a portion of a longer scale extending all the way up the octaves, from the lowest limits of hearing to the highest. Our portion, covering two octaves, starts on C because most melodies are oriented around C (do), as we’ve just explained.

The following diagram shows another portion of the diatonic scale, starting on A (la), because another class of melodies in Western music is oriented around A, not C:

Look carefully at the diagram: Moving up through the octave from C to C, you encounter a different sequence of whole and half steps than you do moving from A to A. This difference gives melodies oriented around A a quality different from those oriented around C. The term for these different ways of centering or organizing the diatonic scale is modality; the different home pitches are said to determine the different modes of music. Music with the do (C) center is in the major mode. Music with the la (A) orientation is in the minor mode.