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Tech Stuff - Jacks and Modular Connectors

We cover here Jacks and Modular Connectors, Telephone wiring standards and LAN wiring standards.

What's in a Name

It's complicated. Once upon a time, when the world was young, we called everything a jack. Pre-1984 (US MFJ) the phone company was the only game in town and owned everything so we couldn't connect squat (forget this personal computer stuff - still largely a business phenomena in those days). We (the heaving masses) had no need for a universal language to describe this connector stuff - the generic term jack was good enough. Then the world changed (post-1984 'ish) and suddenly we were able, and desperately wanted, to connect our stuff.

But connectors have two parts - a thing you plug in, and a thing you plug into (note the highly developed terminology in this description). But terminology evolved, much of it inherited from the phone company, and we started to call the thing we plugged in the male jack (in polite company, sometimes just the jack), and the thing we plugged into the female jack (in polite company, sometimes the wall jack). A tad sexist. Today, almost universally, we call the thing we plug in The Plug and the thing we plug into The Receptacle.

But these connector thingies come in different sizes, typically with 4, 6 or 8 individual wire connectors (you will occasionally come accross some with 2 wire connectors, but these are as rare as hen's teeth). Today, these connectors are called Modular Connectors and differentiated by the number of wire positions they have. Thus, a Receptable with 6 wire positions will be called a 6 Position Modular Connector Receptacle and a Plug with 8 wire Positions would be called an 8 Position Modular Connector Plug. Phew.

So what does the term RJ, as in RJ11 or RJ45, refer to? Technically, RJ (Registered Jack) defines how the pairs are formed in the Plug and Receptacle (see Diagram 2 below). Thus, the term RJ11 can be applied to a 4, 6 or even 8 Position Modular Connector. The table below shows the most frequent use of an RJ designation and its (more) correct terminology. So why is RJ45 called RJ45 when the pair formation is completely different. Who knows, who even cares. It's wrong, OK. But everyone uses the term.

Common Name Correct Name No of connectors Notes
RJ11 4 Position Modular Connector (4PMC) 4 mostly used in analog (telephone) wiring
RJ11 or RJ12 6 Position Modular Connector (6PMC) 6 more commonly used these days in analog (telephone) wiring.
RJ45 8 Position Modular Connector (8PMC) 8 Used in telephone wiring (ISDN and T1), LAN (10baseT and 100BaseT) and RS232 (RS232D) wiring

NOTE: Plug (Male) Modular Connectors are numbered LEFT to RIGHT when viewed from the TOP (TOP is when the plastic lever is on the bottom). Receptacle (Female) Modular Connectors are numbered from LEFT to RIGHT when viewed from the FRONT. An RJ45 (or 8 Position Modular Connector - 8PMC) example for both Plug (Male) and Receptacle (Female) are shown below (the same principle applies to all modular connectors)

Male Connector Numbering

Female Connector Numbering


Diagram 1 Modular Connector Position (Pin) numbering

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Telephone Wiring

Telephone Wiring is defined by USOC (Universal Service Order Code) which is now maintained by the TIA and uses the following conventions for cable pairing.

Analog Line Pair Numbering

The following diagrams show the various ways that single line pairs may be taken from a range of jacks.

NOTE: All numbering is viewed from the TOP (TOP on a Modular connector is when viewed with the lever/tab on the bottom). The diagrams show all possible types that will satisfy the Wiring Code.

USOC Code No of Pairs Diagram(s) Notes
USOC RJ11 or RJ11C


USOC RJ11C A single Pair is formed from positions (pins) 2 & 3 in 4PMC, 3 & 4 in 6PMC or 4 & 5 in 8PMC.
USOC RJ14C or RJ14


USOC RJ14C Two Pairs are formed; Pair 1 from positions (pins) 2 & 3, Pair 2 from 1 &4 in 4PMC; Pair 1 from 3 & 4, Pair 2 from 2 & 5 in 6PMC; Pair 1 from 4 & 5, Pair 2 from 3 & 6 in 8PMC.
USOC RJ25 or RJ25C


USOC RJ48 or RJ48C



Diagram 2 RJ Pair Formation in Modular Connectors

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LAN Wiring

LAN Wiring is defined by EIA (ECIA)/TIA and uses the following two conventions for RJ45 (8 Position Modular Connector) pairing on UTP cable. See our LAN wiring page.

EIA/TIA wiring Code Diagram
568A (colour diagram) RJ45 568A
568B (colour diagram) RJ45 568B

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RS Alphabet Soup
Cable FAQ
Modular Jacks
Serial Primer
RS-232 wiring
lan wiring
trouble shooting lans
mixed lan wiring
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