A burial vault encloses a coffin on all four sides, the top, and the bottom. Modern burial vaults are lowered into the grave, and the coffin lowered into the vault. A lid is then lowered to cover the coffin and seal the vault. Modern burial vaults may be made of concrete, metal, or plastic. Because the sides of the burial vault are attached to the bottom of the vault, the burial vault is generally stronger than a burial liner. Some burial vaults reverse the construction, so that only a base is placed beneath the coffin. The lid consists of the four sides and the top. These types of burial vaults allow a better seal between the lid and base.