You had to ask that!!!! Damn you!!!;-)

Given that you've made it this far in life without knowing the answer to that question--you may ask yourself two things--1)"why haven't I gleaned the answer to this from years of casual conversation with my musical friends?" and 2) "Am I willing to make the effort it takes to make sense out of what is basically the music theory version of the blind men and the elephant?"

But seriously, this question has a big, technical answer, and it's not something that can be spelled out in a half a paragraph by someone who really should be putting away laundry, or dishes, or maybe driving over to the Home Depot to get some hardware to repair something he's been meaning to fix since last February.

Anyway, the answer, which is pretty much the central idea underlying music theory from the before Bach to slightly before Stravinsky really deserves a thread of it's own. Give Jack Campin or Stringsinger, or a few of the others who like to expound on this sort of thing a
crack at it...or you can go to Julliard and see what they know....either way.