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. A burial liner is similar to a burial vault, but does not have a bottom. With a burial liner, the coffin is lowered directly onto the earth. The burial liner is then lowered over the coffin. Modern burial liners may also be made of concrete, metal, or plastic. Many come in a wide array of colors, even stripes. Burial vaults do not prevent the decomposition of the human remains within. Vaults which are installed incorrectly and too tightly sealed may not allow gases generated by the decomposing body to escape. Pressure then builds up within the vault until the vault ruptures, causing the vault to fail. Although some manufacturers of burial vaults claim that their vaults are "green" (environmentally friendly) and prevent the toxic chemicals used in embalming from leaching into the surrounding soil, such claims are uniformly false since the vault cannot be hermetically sealed without causing it to rupture from the pressure of decomposing gases. A truly "green" or natural burial does not use embalming fluids, and does not attempt to protect the body from the soil and rapid decomposition. Modern burial vaults often come in a variety of styles, which can greatly increase the cost. Modern vaults and liners sometimes are lined on the inside with bronze, copper, fiberglass, or stainless steel sheeting, and some vaults and liners are inscribed on the outer surface with words, scenes, or other images. Some jurisdictions require the use of a burial vault or burial liner. For example, several U.S. states require them. In some cases, cemeteries require the use of a burial vault or liner, although it is not a legal requirement. World usage Data on the use of burial vaults and liners outside the