Nobody wants to make an error when measuring for replacement windows. Let's face it - they're large, fragile, expensive, and sometimes extremely rare. Here are the measurements we take: 

 

Outer Frame Edge to Edge

The outside edge of the frame.

Inner Frame Edge to Edge

The inside edge of the frame on the perimeter of the gap between the frame and window sash. 

Glass & Sash Edge to Edge

Edge to edge of the glass & metal sash/trim bordering it. 

Interior Edge

Primarily for fixed flat & vista view windows, this is the measurement of the portion of the window that sits inside the rough opening.