Containers, also known as intermodal containers, are the main type of equipment used in intermodal transport, particularly when one of the modes of transportation is by ship. Containers are eight feet (2438 mm) wide by eight feet (2438 mm) high. The most common lengths are 20 feet (6096 mm) nominal or 19 feet - 10½ in (6058 mm) actual, 40 feet (12192 mm), 48 feet (14630 mm) and 53 feet (16154 mm). They are made out of steel and can be stacked on top of each other (a popular term for a two-high stack is "double stack"). 53' Containers are more commonly referred to as "domestic intermodal containers" as they are usually too large for overseas shipping. NACS containers ("North American Container Services") are a shared fleet of containers, runnable on any TTX or BNSF intermodal lines.