Before replacing your windows, it’s a good idea to familiarize yourself with all the terminology. This way you can make the appropriate measurements and be able to clearly communicate with a dealer or contractor.
OSM is the outside measurement of the window frame and refers to the height and width of the full frame. This measurement is required when measuring for a full replacement window unit.
ISM is the inside measurement of the window frame and refers to the height and width of the inside of the window frame or existing sash. This measurement is required when replacing the box unit of an existing window.
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