An optical fiber adapter or uniter is used to mate two connectors together, usually mounted in a distribution panel or wall box. In most cases it is the critical part of the system that aligns the ferrule part of the connectors keeping the fibres in perfect alignment.

Generally there are two types of alignment sleeve within an adapter, phosphor bronze or ceramic. The phosphor bronze are for general purpose use, the ceramic versions are normally only used in singlemode low loss environments.

In the case of the MT-RJ the alignment is carried out by two small pins fitted to one of the connectors, the adapter is purely to support and lock the connectors together in the distribution panel or wall box.