Width: Measure between inside square edges of outside casing at three points: head, meeting rail, and sill. Use the smallest measurement.
Height: Measure from bottom of square edge of top casing to top of sill in front of each blind stop on the jambs. Use the smallest measurement.
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 the drop sill has or has not been included in overall height dimension.