To enable communication, two must light up simultaneously.

Most of the thousands of human languages use patterns of sound or gesture for symbols which enable communication with others around them.

While not all elements are required, at least two terminals are required in order to enable communication between two people.

Server farms are typically collocated with the network switches and/or routers which enable communication between the different parts of the cluster and the users of the cluster.

This communication might include such things as access authorization information; call pricing information; or other important data necessary to enable communication between the two administrative domains.