Container glossary
What is a twist-lock?
A twist-lock is a locking device with a rotating head that enters the opening of a container’s corner casting and turns a quarter turn to hold it. It is what fastens a container to a truck chassis, a rail wagon, a crane spreader, a ship’s deck and the container below it.
A quarter turn inside the casting
Every corner casting has an elongated opening in its top or bottom face. The head of a twist-lock is the same elongated shape: it passes through when aligned with the opening and cannot come back out once turned 90 degrees. Four of them, one per corner, hold the box against lifting, sliding and tipping.
The same principle comes in three forms. On a road chassis the lock is built into the frame and turned by hand with a lever. On a ship, loose twist-locks are fitted between the tiers of a stack; most are semi-automatic, locking by themselves as the container is lowered and released by a stevedore with a pole. On a crane spreader the four locks turn together under power, and the driver gets a signal when all four have engaged.
On the chassis, in the stack, under a site-built stack
A buyer or a shipper meets twist-locks mainly on the truck. When a container is delivered on a chassis, the driver releases four levers before it is lifted off and locks them again after a box is set down. If a container will travel on a trailer you have arranged yourself, check that the trailer has locks at the right spacing for that length.
The other place is a stack on land. Two containers set one on the other for storage or a building should be joined at the corners; loose twist-locks made for the purpose sit between the castings and are the removable alternative to welding. Stacking practice is covered in the foundation guide, and the standard behind the fittings in the ISO standards guide.
Trusting the weight of the box to hold it
The mistake is thinking a container weighing several tonnes will stay put by itself. Weight resists nothing sideways. An unlocked container on a trailer can shift or come off in a sharp turn or an emergency stop, and an upper container resting loose on a lower one can be moved by wind when empty. People are killed this way; it is the reason every handover includes a look at the levers.
Two smaller errors follow from the geometry. A twist-lock that is only half turned looks closed from a distance, so check the lever position, not the look of the head. And locks only work casting to casting: a 40 ft container cannot be locked onto a single 20 ft below it, because the upper box has no castings at mid-length.
Frequently asked questions
How many twist-locks hold a container?
Four — one in each bottom corner casting. A 40 ft chassis that also carries 20 ft boxes has extra locks at mid-length so that each twenty is held at its own four corners.
Do twist-locks fit every container?
Yes — every ISO container has corner castings with openings of the same size in the same positions, so one design of lock fits a 20 ft dry box, a reefer or a tank alike. That shared fitting is what makes containers interchangeable.
Can I use twist-locks to stack containers on my own site?
Yes — loose stacking twist-locks placed between the corner castings are the standard removable way to join two containers. The boxes must be aligned corner on corner and the lower one must sit level.