Duplex Fiber optic couplers and adapters are used to connect two fiber assemblies together (2 fibers each). When connecting like fiber assemblies together these are referred to as couplers and when connecting two unlike fiber assemblies together these are called adapters. Note that these are offered in Multi-mode and Single-mode options. Single-mode parts will work with both Multi-mode and Single-mode fiber, but Multi-mode versions will only work with Multi-mode fiber.