I'm having trouble thinking of a good example of this (I have a good example, but it doesn't make sense without context.)
Take this silly premise for the sake of explaining what I'm talking about: Imagine for a moment that whenever someone talks about a shape, the shape that ALWAYS springs to mind is square. Squares represent almost all shapes in people's minds, so when you talk about a shape, you might as well be talking about a square. In this situation, a square is the _ example of a shape. Archetypal?
(I'd also be interested if anyone else has good examples -- that's better left for a comment)