I'm also going to get exceedingly pedantic here.



When you see the letters "RJ" in front of a number, like "RJ11" and "RJ45", that does NOT refer to the actual connector. RJ stands for "Registered Jack" and is defined in the Uniform Service Ordering Code, and refers to a specific type of circuit provisioning - so when the phone company dispatched a tech to the field to install a circuit, they would specify an RJ code, which told the tech a few things:

• What type of connector was required (typically a "modular connector" that we're all familiar with)

• The pinout of that connector (and how many contacts the modular connector needed)

• What type of circuit it was



RJ11 defines a single telephone line on the two center pins of a 6-position/2-contact (6P2C) modular jack. An analog telephone requires RJ11 configuration, and as such has a 6P2C modular plug on the end of its cord.



RJ14 is a similar spec for either a dual analog line (line 1 on the middle 2 pins, line 2 on the outer 2 pins), this time on a 6P4C jack/plug. This was also used for digital PBX handsets that required separate pairs for power and signal.



RJ25 a three-pair phone specification on a 6P6C jack/plug.



all the 6P connectors are interchangeable.



The handset connection on most phones is a 4P4C plug but is not used by any RJ definition.



RJ45 was a single-pair data circuit specification on an 8P8C connector with the data on the center two pins, and a resistor between pins 7 and 8 (where the brown pair goes on TIA/EIA 568).



There are about a dozen other RJ specs that use the 8P8C modular connector (RJ31X being very common for alarm panel connections (in fact, your connection block above shows that as "line seizure). Since 8P8C with TIA-568A/B was also commonly used for Ethernet, the connector itself over time began to be referred to (incorrectly) as RJ45.



TIA-568A is wired to be compatible with RJ11 and RJ14 (as shown above, you can put a 6P modular connector in an 8P modular jack).



From a terminology standpoint, RJXX is equivalent to TIA-568A/B in that it defines the specific XPXC wiring scheme rather than the connector itself (although by defining the wiring scheme, it specifies a particular connector)