With apologies to everyone who, like myself, is irritated by a thread being posted on different boards, I paste here the thread I just started on another board. (I am so ashamed.)
There are several narratives of twentieth century music, none of them quite complete, but maybe no narrative is ever quite that.
The commonest, and the one implied in most posts about modern/avant garde/atonal/contemporary music, goes something like this:
Sometime early in the century, Schoenberg made a radical break with the past, turning his back on tonality and all that implied and ushering in an age of academic dissonance and atonality, which alienated audiences everywhere.
While serialism had a stranglehold on new music in the fifties and sixties, by the last couple of decades of the century, composers had rejected the intentionally ugly and inaccessible sounds of atonality for more audience friendly music. Tonality returned and by the twenty-first century, young composers were pluralists, using patterns and techniques from every era without dogmatism.
A more detailed version of that might mention the populism of the thirties and forties, the minimalism of the sixties and beyond and would emphasize the contributions of non-serial composers like Shostakovich and Pettersson. The bulk of commentary would be heavily weighted toward the early years of the century, Debussy, Mahler, Sibelius, Stravinsky and the like.
Even more well-known and popular is the narrative that starts the century with rag-time and moves into various kinds of jazz and Hollywood (easy listening) musics before 1950 and moving into rock and roll and then a bewildering variety of pop and c & w and hip-hop and metal and prog rock and the like by century's end.
Neither of these narratives has anything to say about experimental or electroacoustic musics, though the words "experimental" and "electronic" have been co-opted by the popular music world, without any reference to their original designations. To get a narrative about those things, one has to look at a university textbook. Though the bulk of detail will still be about the early decades of the century, books for music classes will mention musique concrete and electronic music and possibly even synthesizer music and will also mention a few things about experimental music, though there might be some confusion even there about indeterminacy and aleatory. (They're not synonyms.)
The second narrative needn't occupy our time very much, I don't think. Not on this forum, anyway, though a more complete narrative of art music would certainly include the people in rock, jazz, and classical traditions who had common interests and produced similar sounding results, people who often as not knew each other and worked together as well. A more complete narrative would talk about the phenomenon of collaboration, for that matter, which by century's end had become standard behavior for a wide range of new music musicians.
A more complete narrative, what's more, would cover more of the century's prominent opposing pairs than tonality/atonality, which for many musicians was stale by 1939.
Starting with 1939, a more complete narrative would include noise/music, intention/non-intention, simplicity/complexity, electroacoustic/electronic, acousmatic/soundscape, through-composed/improvisation (live electronics).
The relative values of both composers and trends would change, too. Dodecaphony and, later, serialism were very important, but they were never ever the only thing, even at Darmstadt (where Cage and Tudor visited in the fifties). Things like Fluxus would be known and perhaps even be seen as closer to the center of things than some fringe activity that never really caught on. Dhomont and Ferrari and Radigue and Marclay would be much better known as well, as well known and as revered as Ligeti and Xenakis and Berio are now.
Online conversations would no longer be dominated by ancient controversies (like the tonal/atonal one) and might even be able to eschew simple generalizations like the one about the dominance of serialism in the fifties and sixties. (Britten, Shostakovich, Vaughn-Williams, Jolivet, Barber, Dutilleux, Part, and Harris were all writing in the fifties and sixties, too, among other equally non-serial composers.)
Hey! It could happen!!