Yes, English is my first language. I don't know why I internalized that symbol the way I did. And looking at the Atari TOS screenshot, I would have said the symbol in the upper right corner looked like an elevator "open" symbol, something like <>. But thinking more deeply, >< and <> are in only one dimension. They are good for expressing the idea of "close" and "open" respectively, but they fail to acknowledge the two-dimensional character of a window. So, I like what some of the other posters have said about four arrowheads pointing in and out respectively. That said, the symbol of two diagonal, outward-pointing arrows in the upper right corner of Mac OS X windows now strikes me as brilliant; in a minimal way, the idea of maximizing in two dimensions is expressed.

Actually there are buttons with a line through them, and ones without. They maddeningly mean different things. See http://petesguide.com/symbols/elevator-button-symbols/