I’m an architect, and I’m told by my window rep that double hung windows are measured from inside the frame while casements are measured from outside, so that a 3′ nominal casement will be narrower than a 3′ nominal double hung.  In previous schedules I’d been noting everything from outside the frame, even double hung, and the rep just interpreted my intentions and made it work.   But we recently had a job where some double hung windows in an existing brick wall had to be replaced with casements, and there was some confusion about whether the nominal casement sizes would be large enough to fit the openings with a brick mold that matched the other double hung windows, so I’ve now been trying to observe the distinction in how I call out the two types.   But since I haven’t heard anyone else talk about this in 6 years of experience, I wonder if I’ll cause anyone else confusion.  Have others encountered this distinction?  Do all window companies do this?  Anyone run into problems with ambiguous measurements because of this?