Surely it's the MUSIC that defines an era, not the instruments upon which it is played ? If you're looking for something that defines the 20thC, you're going to have to be a deal less eclectic. At the start of this century, into the 20's, there was a deal of variety, from chamber and classical to the birth of the 'jazz-age'. What instrument 'defined' that era ? Woodwind, I venture to suggest. We're moving forward here....and, hey!, guess what? The 'Big Band' era is with us and, yes, it's still woodwind! Lump into this equation the advent of the steam-driven radio, the more general availability of recorded sound, and NOW you have a move towards more 'populist' types of music. With the birth of the 'pundit' came all sorts of complications, but it only relatively recently that the guitar has become 'popular' - even if you choose ignore the classical works. And what has been at the fore of ALL these musical styles? GeorgeH had it.......the voice....and even that was used as a method of interpretation!!