The recess decides it. Usable depth — from the sash out to the front edge of the trim, minus anything sitting in that space — and the squareness of the opening are what tell you whether a headrail can disappear inside the window frame or has to sit on the wall above it. Both numbers get written down before anything is ordered, because the mount type you choose determines the width you put on the order form, and nothing you do at the window later can undo that.
I keep a step stool, a 25-foot tape, a stubby torpedo level, and a scrap of corrugated cardboard in a bucket by the back door. That bucket has settled more blind decisions in my house and in three neighbors' houses than any showroom ever did. Ten minutes per window, and the scene is always the same: knee on the sill, shoulder braced against the jamb, phone flashlight angled up under the top trim because the recess is a shadow you cannot see into from standing height.
Do that once and you stop thinking of this as a style question.
The depth you can use is not the depth you can see
Measure from the glass or the sash straight back to the front edge of the trim. That gives you a raw number, and the raw number is almost never the one that matters.
Now subtract the obstructions. A casement crank handle. The tilt-in latch — the little plastic shoe at the top corners of a double-hung sash. A storm window frame. The screen channel. The lip where the sill meets the jamb. Inside mount needs room for the headrail plus the mounting brackets behind and beside it, and the honest rule of thumb is somewhere in the neighborhood of one to two inches of clear depth, with the smaller end of that belonging to slim one-inch aluminum and the bigger end to two-inch faux wood with a valance or a continuous cord loop.
So don't shop by how deep your window is. Shop by how much depth is left after you've accounted for everything standing in it. A window with three inches of recess and a crank handle sitting in the middle of it has less usable depth than a window with an inch and a half and nothing in the way.
Squareness fails windows that depth passed
Width at the top of the opening, width at the middle, width at the bottom. All three, written down. The narrowest one governs — the blind has to pass through the tightest part of the opening, not the average of it.
A quarter inch of variation across a three-foot window is normal in an older house and won't kill an inside mount. Half an inch or more is where it goes wrong: the blind binds at the narrow end, or it sits fine and leaves a light stripe at the wide end that reads as a mistake from across the room. You can't shim your way out of a trapezoid.
While you're up there, set the level across the top of the recess. A headrail that's obviously out of level inside a frame is more noticeable than one that's slightly off on a wall, because the trim gives the eye a straight line to compare it against.
Set a piece of cardboard in and look at it
Here's the test that ends the argument. Cut a strip of cardboard as deep as the headrail plus its bracket — for a typical two-inch faux wood inside mount, that's around two inches. Hold it flat against the underside of the recess at the top, where the headrail would actually live. Not in the middle of the opening, where there's always plenty of room.
Then look at two things. Does the sash clear it? And is there still enough space in front of it to reach a cord or a wand without skinning your knuckles?
Doing this physically beats reading a spec sheet, because a spec sheet describes the product and you're measuring a specific hole in a specific wall. If your cardboard strip has a finger's width of clearance and the sash moves freely, an inside mount is real. If it's touching, you're at the line, and the line is where the sash hits in February when the humidity changes.
Why a tilt-in sash is usually the verdict
The single most common reason a window that measured fine ends up going outside mount is the tilt-in sash. The bottom sash of a double-hung tips inward for cleaning, and the latch hardware sits right at the top corners — exactly the pocket an inside mount headrail wants to occupy. If the recess is shallow, one of two things happens: the sash won't tilt anymore, or it tilts and drags across the headrail every time somebody washes the glass.
I've watched people discover this after the blinds are up. There's no adjustment for it. The blind is already cut to width and you're not sending it back.
Crank handles on casements and awnings do the same thing from a different direction. So does a storm window panel that sits proud of the jamb.
Outside mount has a requirement people skip
Outside mount attaches to the wall, the trim, or the casing above the opening, and it should overlap the opening on both sides and along the top to cut the light leak, running down to the sill or past it at the bottom. The overlap is the whole point. Mount it flush with the opening and you've built a light leak with extra steps.
What people forget to check is the surface it lands on. Run your palm across the wall above the window. You need a flat, solid surface wide enough for the brackets to spread out and flat enough that the headrail doesn't rock. On a cased window, the casing is typically proud of the drywall by half an inch or so, which means the headrail sits on the casing, not the wall — that's fine, but you need to know which plane you're building off, because it changes how far the blind projects into the room.
On a window with no casing, where the drywall returns tight to the opening, you may be looking at a narrow strip of drywall and a corner bead and not much else. That gets dealt with before anything is ordered, because the bracket spread and the projection both come off that surface.
The reason this is a pre-order decision
Inside mount and outside mount are ordered at different widths. Inside mount goes to the narrowest measured width, often with a small deduction. Outside mount goes wider than the opening so that it overlaps.
That means the size you write on the form is welded to the mount type. A blind sized for inside mount and hung outside leaves the edges of the opening exposed — you've paid for the light control and thrown it away. A blind sized for outside mount and forced into the recess doesn't fit through the opening in the first place.
There's an honest tradeoff in here too, and it cuts both ways. Inside mount is the tidier look: the headrail disappears, the window reads as architecture instead of hardware. But it leaks light around the edges by design, because there's no overlap. Outside mount kills the light leak, which matters if this is a bedroom or a nursery, and in exchange it takes up wall space and adds bulk above the trim that you'll see from every angle in the room.
When it genuinely doesn't matter, and when you're not choosing at all
If the recess is deep, nothing is standing in it, the opening is square within a quarter inch, and the wall above is flat and solid, then you're choosing on looks. Inside for clean lines. Outside for darkness. Neither is wrong.
If the recess is shallow and there's no solid surface above the opening — which happens on some newer builds where the window is set into drywall with a return and no casing — you're not choosing a mount type. You're solving a surface problem first, and the mount type follows from that.
And one more check before you commit to inside mount: make sure the headrail will actually sit behind the plane of the trim. If it ends up proud of the trim, you've got the visual bulk of an outside mount with the light leak of an inside mount, which is the one outcome with no upside.
