Defining twentieth-and twenty-first century music

What do we mean by ‘twentieth-century music’? And how are we to square this with the musics of a twenty-first century that is now nearing the end of its second decade? These and other questions are salient for a journal that identifies the former century in its title yet regards the latter as equally within its remit. Just how are we to think the two centuries together? Should we consider the music of the twenty-first century as a continuation of tendencies from the late twentieth? Or are there tendencies within musical production and consumption that have a definitively twenty-first-century character and so mark out the beginning of a new era? If so, when and with what, iconically speaking, did the twenty-first century begin and the twentieth end? Or do these historiographic categories even continue to have currency?