RJ9
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The connector incorrectly known as RJ9 is the unofficial industry standard for wired telephone handsets. It is used to provide connection from the base of the telephone to the handset. "RJ" stands for Registered Jack, a family of telephone connection standards. This handset connector is actually not a Registered Jack at all, since it was never intended to connect directly to the telephone service lines. It uses the 4P4C modular connector, which has four pins or electrical connections per connector.
USOC registered jacks RJ9 | RJ11 | RJ12 | RJ14 | RJ21 | RJ25 | RJ45 | RJ48 | RJ50 | RJ61 |