I have been doing a bit of reading lately about "Twentieth century" music. The Twentieth century title really describes music from the early 1900s to present day, so perhaps historians need to come up with a few new labels. Anyway.
A big part of the Twentieth century revolution in music is something called atonality. You may surmise, from the "a" in front of "tonality", that atonal music is not tonal. You would be correct. But, what does that mean? Let's look a bit closer at atonality, because I don't want to you be lost later.
Tonality is what we understand best (or, at least, it is what we are most accustomed to hearing). Pop music is tonal, folk tunes and nursery songs are tonal, much of the Classical genre is tonal. By calling something tonal, we really mean that there is a pitch that the music centers around. It is scalar and diatonic in derivation and harmonic function. In short, stuff works in a fairly predictable way. A V chord will typically make its way to a I chord, without much weird motion in between. And even if there is some unusual harmonic motion, the resolution will arrive, even if it is delayed for some reason.
I picture tonal music like this:
Try it now: Do re mi fa sol la ti
If you grew up surrounded by tonal music, as many people do, you may experience a sensation of being unsettled. We "want" that final do to sound. You probably silently gave yourself the resolution anyway. I always do, even if I don't vocalize it. Tonality is like coming back home after a long vacation.
Atonality is different. There is no tonal center. No gravity. It is kind of like this:
