I'm looking for a noun to describe the situation where something is changing width. Have a look at this hypothetical window frame:
Window Frame with Two Panes of Uneven Height
The bottom part of the frame changes width across the window. What might I call the category of windows where this occurs?
I have thought about "widening," but am not overly fond of it, since it might go from wide to narrow, or you could even have a situation with more panes, where it is wide-narrow-wide. Any ideas?