To save space in container yards, containers are generally stacked on top of each other in containers stacks.
The goal of the container yard operator is to minimize the number of moves required to access and load the containers on the departing means of transport.
In our below picture, container 1 and 2 will for example be leaving on an earlier sailing than 3 or 4 and therefore, when they (1 and 2) arrived on container chassis, they were stacked on top of already present later departure containers (3 and 4) [simplified example].
