I realized I need to say a little more about Modernism. Movements do not spring up in a vacuum. Harold Bloom, in his brilliant book The Western Canon, posits literature as a dialectic - or argument - the geniuses of one generation fighting off the influence of the previous generation by creating something new. I love this idea and find it useful when thinking about any part of culture. So...
Modernism was a deliberate attempt to distance the arts of the 20th century from any work that had come before. The screaming pace of industrialization and the brutality and loss of the first world war, led artists to decry the effete and upper-class-feeling decorativeness of the immediate past, not to mention THE PAST in a general way. Pared down, egalitarian, daily experience, communicative - these were the sought-after qualities. Nothing was meant to feel "styled" for effect.
Then...Post-modernism is a response to Modernism. For Post-modernists, Modernism is a style, too, despite its anti-style stance. All of the work of man can be seen to be constructed within a style paradigm, even if that style paradigm says it is about plain-ness. Therefore it is an historical style against which an artist can position his or her work, and can be referred to together with any other historical style in a world view that encompasses the idea of culture as a construct.
The marigold wonders nervously:
that does make it more clear, doesn't it?