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If you are doing a Keystone jack installation, it's helpful to know what "RJ" means because you will see it a lot when referring to wiring jacks.
"RJ" means "registered jack." It sounds simple enough, but it's actually become a complicated term in the industry. That's because it is often used incorrectly.
You will see numbers like RJ-11 and RJ-45 to refer to jacks. These used to refer to different wiring schemes for modular jacks. Today, we still use the RJ configurations, but not in the original way they were intended by the Universal Service Ordering Codes (USOC) back in the day. And this can be confusing.
Therefore, industry experts say the most accurate way you can identify a jack, and leave no room for misinterpretation, is to leave out the RJ and instead specify the number of positions (width opening) and conductors on the jack. For example, a "6-position, 4-conductor" jack.