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A standard tetrapolar telephone plug in electronics and telecommunications, has four round metal pins and one plastic pin. The design is only used in Belgium for telephone wiring. [1] It is similar to the tripolar telephone plug of Italy and also the Swedish telephone plug.
The plastic pin adds a presence function. When not inserted into a jack, the jack itself (mechanically) connects the incoming line to the next socket. The article on the Swedish telephone plug details its functional principles.