"Me" The contract on our house says,
"All windows casement except bay flankers".
To me, that means every window is a casement window that opens. The
builder
claims that every window is a casement window, though most of them
(curiously, not all) have a fixed section of glass (pane, window, ?) and a
casement section.
Our current house has similar windows and both sides open in every window
large enough to be split. I find myself referring to the entire opening as
a
window, but then referring to the individual sheet of glass as a window as
well, so I can see the argument from the builder's perspective while
knowing
what I intended when we ordered the windows. Can anyone clarify for me?

FWIW: The windows are Andersen and when I look at their web site, they
don't
even show casement windows with a fixed side!

I am just guessing, but in absence of a casement window definition in your
contract, he may have you by the short and curlys. I am guessing that any
window unit with a casement pane would, in some stretch, qualify as a
casement window. Normally you would be provided a model home, or a detailed
architectural plan with window spec pages, to get graphic confirmation of
what kind of window is going in. Going by words alone is always a risk, in
absence of those references.