Why does a cushion look flat? Slide the insert and see

A cushion looks plump or tired mostly because of its insert, not its cover. Slide an insert from smaller to bigger inside a cover and watch the corners fill, then strain.

You'll see why an insert about 2 to 5 cm wider than the cover is the usual rule, how feather, polyester fibre and foam change it, and how to measure your own cushions before you buy anything.

Plus a few cheap ways to bring a flat cushion back to life.