Blister packaging involves preformed plastic blisters, heat-seal coatings, and a backing such as paperboard, plastic or foil. Generally the items are placed into the blisters, then the heat-seal coating and backing are applied. Heat binds the two layers of material. Pills and makeup are commonly packaged in blister packaging. A clamshell is formed when a single layer of blister packaging has a hinge allowing it to fold over on itself. Nails and earphones are often packaged in clamshell packaging. In skin packaging, items are placed on a cardboard backing, and a thin layer of film covers it. Then the film is vacuumed onto the item and the cardboard backing. Skin packaging is similar to a blister pack, with the main difference being that the cavity for the product is not pre-formed. Skin packaging is often used to package hardware items.