Coffins are traditionally made with six sides, tapered around the shoulders, or rectangular with four sides. Continental Europe has favoured the rectangular coffin or casket, although variations exist in size and shape. In Medieval Japan, round coffins were used,which resembled barrels in shape and were usually made by coopers. (In the 1961 Kurosawa film Yojimbo, the protagonist, anticipating a shortage of coffins due to an impending battle (planned by Yojimbo) persuades several coopers to start making more coffins.) In the case of a death at sea, there have been instances where trunks have been pressed into use as a coffin.
They may incorporate features that claim to protect the body or for public health. For example, some may offer a protective casket that uses a gasket to seal the casket shut after the coffin is closed for the final time. In England, it has long been law that a coffin for interment above ground should be sealed; this was traditionally implemented as a wooden outer coffin around a lead lining, around a third inner shell. There are occurrences of coffins lined with or constructed from lead to bury radioactive-contaminated dead. However, in practice, after some decades have passed, the lead may ripple and rip. In the United States, numerous states require a vault of some kind in order to bury the deceased. A burial vault serves as an outer enclosure for buried remains, the coffin serves as an inner enclosure.
Some manufacturers offer a warranty on the structural integrity of the coffin. However, no coffin will preserve the body, regardless of whether it is a wooden or metal coffin, a sealed casket, or if the deceased was embalmed beforehand. In some cases, a sealed coffin may actually speed up rather than slow down the process of decomposition. An airtight coffin, for example, fosters decomposition by anaerobic bacteria, which results in a putrefied liquefaction of the body, and all putrefied tissue remains inside the container, only to be exposed in the event of an exhumation. A container that allows air to pass in and out, such as a simple wooden box, allows for clean skeletonization. However the situation will vary according to soil or air conditions, and climate.
Coffins are made of many materials, including steel, various types of wood, and other materials such as fiberglass. There is emerging interest in eco-friendly coffins made of purely natural materials such as bamboo, willow or Banana Leaf.
Custom coffins are occasionally created and some companies also make set ranges with non-traditional designs. These include printing or painting of peaceful tropical scenes, sea-shells, sunsets, cherubs and patriotic flags. Some manufacturers have designed them to look like gym carry bags, guitar cases, cigar humidors, and even yellow dumpster bins. Other coffins are left deliberately blank so that friends and family can inscribe final wishes and thoughts upon them to the deceased. In Taiwan, coffins made of crushed oyster shells were used in the 18th and 19th centuries.