Counterpoint and Harmony.

If you play piano, guitar, any fully 'homophonic' instrument, start with Harmony. If you play a monophonic instrument try Counterpoint.

I think you mean "polyphonic instrument" when you refer to guitar or piano, no?

either or I guess, homophony is multiple parts moving together to form harmony right?

It was fairly obvious what he meant, but I'm not sure if I agree that the instrument you learn should dictate what theory you study. It will almost inevitably influence how you perceive musical construction anyway.

But it could halp with how you understand the theory. I have to put scales to a fretboard before i really "get" them.

It was fairly obvious what he meant, but I'm not sure if I agree that the instrument you learn should dictate what theory you study. It will almost inevitably influence how you perceive musical construction anyway.

For a pianist, guitarist, someone who plays multiple notes on the instrument at once, there is a familiarity with large harmonies and they would have easier time grasping Piston's triads and four part harmony. The book basically gets right into that at the get go. But for smoeone who plays violin, clarinet, trumpet, etc. the aural familiarity may not be there as much, so to work with one line at a time could be easier.