A chain is a series of connected links, usually made of metal. A chain may contain two or more links.

Products specially designed for lifting, such as when used with a crane; ring links that are pulled or used for fixing (e.g. using a bicycle lock) make the chain flexible in both directions (the third dimension fixed is the length of the chain).

Those gears designed to transmit power in the machine have links designed to mesh with the sprocket teeth of the machine, and are flexible in only one dimension. They are also known as roller chains, although there are also non roller chains, such as blockchains.

You can use a quick link to connect two distinct chains, similar to a mountaineering buckle with a screw closure instead of a latch.