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Grease nipples, also referred to as Zerk fittings and grease fittings, are lubricating points connected to machinery where grease guns are attached to introduce grease into the system. These grease fittings are permanently installed in machinery via a threaded or a straight hammer-in connection. They resemble a nut cap with a protruding ball at the tip where grease gun couplers attach to. Grease nipples also serve as a check valve that allows the entry of grease but prevents grease from coming out of the system. A common variation of grease fittings called button-head fittings have a flat head resembling a button require a corresponding button head coupler to attach to and used when large volumes of grease are transferred.