Should I choose an inside mount or an outside mount?

It comes down to depth, squareness, and darkness. Inside mount sits the blind inside the frame for a clean, built-in look — but it needs enough flat mounting depth for the headrail and an exact width, and it always leaves a small light gap at the edges. Outside mount fixes the blind to the wall or trim above the opening; it forgives shallow or out-of-square windows, blocks more light because it overlaps the frame, and is the route to real room-darkening, at the cost of covering the trim. If your frame is deep, square, and you want the tailored look, go inside. If it’s shallow, crooked, or you need darkness, go outside and add 2–3 inches of overlap per side.