A topper or underlay is an extra layer of padding that sits between your mattress and your mattress protector. Made from wool, feather & down, foam or polyester, toppers and underlays provide a more substantial layer of comfort than a mattress protector. Premium woollen underlays can be reversible with one side being wool and the other cotton. In the colder months, use it wool-side-up for extra warmth. Enjoy the coolness of cotton in the warmer months.