What the liner is for, and when to use it

Three layers: a foam core, a waterproof fitted liner over it, and the cover you see. The liner is the one nobody asks about, and it's the one that protects the part you can't replace.

What it is

A fitted sheet, in the literal sense. Soft knit terry, 80% cotton and 20% polyester, laminated to a thin polyurethane membrane, cut to the bed's shape and finished with an elastic edge. No zipper. It pulls on the way a fitted sheet goes on a mattress.

Terry on the side facing up, so it feels like fabric rather than plastic. The membrane on the underside, so liquid stops there.

What it's for

The cover is water-repellent, which buys you time, not immunity. A spilled bowl, a wet dog, a puppy, an older dog having a difficult night: the cover will hold a surface spill long enough for you to notice it. It won't hold a soaked one.

The liner is the layer that means it doesn't matter. Whatever gets past the cover stops at the liner, and the foam stays dry.

One thing to be clear about: the foam doesn't wash. Not cold, not gently, not by hand. It's the one part of the bed that can't be taken out and cleaned, and it's the one part not sold on its own. The cover washes, the liner washes, and both of them exist so the core never has to.

When to use it

Always. It isn't an accessory and it isn't a seasonal thing; it ships with every bed and it should stay on.

There are situations where you'll be glad twice over: a puppy in the house, an old dog, a dog who swims, a dog who comes in off wet grass and goes straight to bed, anything involving an upset stomach. But the routine answer is that the liner lives on the foam and comes off only to be washed.

Washing it

Machine cold, no bleach, no fabric softener. Line dry or lay it flat. Don't put it in the dryer, and don't iron it. Heat is what damages a laminated membrane, so every instruction here is really one instruction: keep it cool.

Softener is worth its own sentence. It leaves a film, and a film on a waterproof layer is exactly what you don't want. Skip it.

Let it dry completely before it goes back on the foam. Damp fabric against foam is the one thing you really shouldn't leave overnight.

Getting it back on

Lay the liner flat on the floor, terry side down. Set the foam on top with the indent of the foam lined up with the indent of the liner, then pull the elastic edge up and around the foam all the way around. Then the cover goes on the same way: lay it flat and unzipped, foam on top, indent to indent, pull the edges up, zip it closed.

If it needs replacing

Liners are sold on their own, in white, in all four sizes. A spare is the cheapest insurance in the range and it means you're never waiting on a wash cycle to put the bed back together.

• The liner.
• The care page has the whole routine, in order.