Tweendeckers are general

cargo ships A cargo ship or freighter is a merchant ship that carries cargo, goods, and materials from one port to another. Thousands of cargo carriers ply the world's seas and oceans each year, handling the bulk of international trade. Cargo ships are usu ...

with two or sometimes three decks. The upper deck is called the ''main deck'' or ''weather deck'', and the next lower deck is the ''tweendeck''. Cargo such as bales, bags, or drums can be stacked in the ''tweendeck space'', atop the tweendeck. Beneath the tweendeck is the ''hold space'', used for general cargo. Cargo ships that have fittings to carry standard shipping containers and retractable tweendecks (that can be moved out of the way) so that the ship can carry bulk cargo are known as ''multipurpose vessels''. Infomar
"Dry Cargo Ships"
Dec. 25, 2006.

References

Ship types {{ship-stub