Measure twice. Order the window you can't return.

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.

The width deduction is the small amount a factory subtracts from your inside-mount width so the finished blind clears the frame and operates freely — commonly 1/4" to 1/2", though it varies by seller and product. The detail that trips people up is who does the subtracting: most sellers deduct for you (you give the exact opening size and they take it off), but a few expect you to deduct yourself, and getting that backwards double-deducts and leaves a gappy blind. This is the most consequential number in a custom order. Confirm against your seller’s measuring guide whether to give the exact opening or a pre-deducted size — their rule wins, not a general one — because custom blinds are generally not returnable for a mismeasure.

Generally, no. Custom blinds are made to your exact measurements, so they can’t be resold, and nearly every seller treats them as final-sale — a wrong size or a color you regret usually isn’t returnable the way a stock product is. The protection that exists is the remake policy: some sellers will remake a blind once for a customer mismeasure (often with a size limit and who-pays-shipping terms), and most will fix a genuine manufacturing error. That fine print is worth real money, so read it before you buy. The practical takeaways: measure twice, order one problem window before committing to a whole house, and treat the remake policy as part of the price you’re comparing.

Because "blackout" describes the fabric, not the installed result. A blackout fabric passes no light through the material itself, but mounted inside the window frame the shade still leaks daylight around its sides and bottom through the deduction gap — the room ends up dark-ish, not dark. This is the category’s most common disappointment, and the fix is structural, not a different fabric: an outside mount with generous overlap (2–3 inches per side), side channels that seal the vertical edges, or a layered day/night setup. If a genuinely dark bedroom or media room is the goal, plan for the mount that seals the edges from the start — the fabric is only half the answer, and the gap is what wakes you at dawn.