Screen sizes describe the diagonal, not the width — which is why two "15-inch" laptops can have noticeably different footprints, and why a monitor that sounds large may not be.
Screens
Diagonal is the stated size. What you usually need instead is:
- Footprint — width and depth of the device or the stand
- Aspect ratio, because a wide screen and a standard one at the same diagonal are different shapes
- Bezel size, for anything going into a fixed space
- VESA mounting pattern, if it is going on an arm
Laptops
Ask for width, depth and thickness, not just the screen size. A sleeve or bag bought on the screen size is the classic mistake in this category.
Phones and tablets
Cases are model-specific, not size-specific. A case for a model that looks similar is not a case for yours — ask for the exact model, including the variant.
Audio
Speaker dimensions and driver size are different numbers. Headphone band sizes vary and are rarely stated; ask for the clamp or the band range if it matters.
Everything else
Weight, because a device you will carry is a different purchase from one that sits on a desk. And for desktops, the case dimensions and the clearance inside if you plan to add anything.
What to ask the seller
"What is the exact model, and can you measure the width and depth?" The exact model answers most sizing questions by itself. Questions to ask.
What protection covers
A different model than described, or dimensions materially wrong: seven days from delivery. A device that did not fit a space you did not measure is not.
Questions people ask
Is 15.6 inches the width?
No — the diagonal of the screen.
Will my old case fit?
Only for the same model. Ask for the model number.
Does the stand count in the depth?
For a monitor on a desk, very much so. Ask for both figures.