When measuring for window shadings, whether they are cellular shades, roman shades, privacy sheers, Silhouettes, or any type of window covering that mounts directly at the window, it’s important to make the right decision as to whether you want to mount the shade inside the window trim (inside mount), or outside the trim (outside mount). The first consideration is whether or not there is enough room on the inside of the trim to mount your brackets. Some trims are either not deep enough, or have too much detail to allow a flat surface onto which you can mount your brackets. If this is the case, an outside mount is necessary. Likewise, if light gaps are an issue (say in a bedroom, where you want total blackout), an outside mount might be the best option, so you can extend the shade 2-3 inches on either side of the opening. This will give you the most light control.
If there is sufficient depth to the window casing, and you prefer a custom fitted look, an inside mount is the preferable option. Assuming the window is “square”, you can achieve this look if you can measure correctly. A professional window fashions consultant can help you determine your options.