Inside or outside mount, decided honestly

Blinds and shades hang one of two ways, and the frame you already have usually makes the choice for you.

What each mount actually is

An inside mount sits the blind within the window recess, flush and tailored, showing off the trim around it. An outside mount fixes the headrail to the wall or the trim itself, so the blind hangs in front of the opening like a small curtain.

The depth test settles most cases

Inside mounting needs a recess deep enough to hold the headrail without it poking past the trim. Measure from the glass to the front edge of the frame. Shallower than about an inch and the hardware will sit proud, catch light and catch elbows. That is an outside mount telling you what it wants to be.

1. Measure the recess depth at the top corners, where brackets go.
2. Check for cranks, handles and locks that swing through the space.
3. If both clear, measure the inside width at the top exactly where the headrail sits.
4. If either fails, mark the wall two to three inches past the frame on each side and mount outside.

Light leaks are part of the deal inside the frame

Even a perfectly cut inside mount leaves a sliver of daylight along each edge, because the blind must clear the frame to move. Bedrooms that need real darkness do better with an outside mount wide enough to overlap the opening, or with a darkening panel layered over the blind.

When the window is out of square

Older frames lean. If the top width and bottom width disagree by more than half an inch, an inside mount will show a wedge of light at one corner no matter how carefully you drill. Outside mounting forgives all of it, which is why it is the default in old houses.