It's kind of like , where you had side A,side B and you just flipped it over when you wanted to change your mood so to speak.

VOA: standard.2010.06.18

These between B and C and E and F didn't get filled in because they were already just the smallest amount that they wanted to deal with, just half steps.

但是B和C,E和F之间并没有被填上,因为它们之间已经是最小的距离,只是个半音级,人们愿意摆弄的最小距离

耶鲁公开课 - 聆听音乐课程节选