Inside mount cornices can be mounted over blinds or shades to cover the headrail of your window treatment. The look is neat, clean and uncluttered. There will be a deduction of 1/4" off ordered width. You also want to consider the thickness of selected fabric which can add from 1/8" to 1/4" to overall width.
Outside mount cornices will add substance to your windows. Cornices have a dustcap and returns that wrap to the wall. The backside and dustcap are covered in a white privacy lining, and cornices are available with 1/2" welt cord along bottom edge.