Standard Fiber Optic Adapters (also called couplers) SC LC FC MU E2000 MTRJ are designed to connect two fiber optic cables together. They come in versions to connect single fibers together (simplex), two fibers together (duplex), or sometimes four fibers together (quad).
Standard Fiber Optic Adapters are designed for multimode or single mode cables. The single mode adapters offer more precise alignment of the tips of the connectors (ferrules). It is ok to use single mode adapters to connect multimode cables, but you should not use multimode adapters to connect single mode cables. This can cause misalignment of the small single mode fibers and loss of signal strength (attenuation).