They are attached to each other and the ship via some mechanism that interacts with their corner castings. (Each corner has a box with slots in it.) Turnbuckles

They are attached to each other and the ship via some mechanism that interacts with their corner castings. (Each corner has a box with slots in it.) Turnbuckles, twist locks, rails, etc are used to secure the containers. See http://www.pacificmarine.net/marine-deck/cargo-securing/container-securing.htm http://www.pacificmarine.net/marine-deck/cargo-securing/cont... for some options.