Width: Measure between inside edges of outside casing at three points: top, meeting rail and sill. Use the smallest measurement.
Height: Measure from bottom of top casing to sill along the face of each side casing, so as to include slope of the sill.
Drop Sill: If sill has drop of over 3/4" (See Drop Sill Illustration, right) specify drop sill height on your order and indicate whether drop sill has or has not been included in overall height dimension.