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This article is about the registered jack (RJ) wiring standard. For other uses of modular connectors, see Modular connector.

A registered jack (RJ) is a standardized[1][2][3] physical network interface—both jack construction and wiring pattern—for connecting telecommunications or data equipment to a service provided by a local exchange carrier or long distance carrier. The standard designs for these connectors and their wiring are named RJ11, RJ14, RJ21, RJ45, RJ48, etc. Many of these interface standards are commonly used in North America, though some interfaces are used world-wide.

The physical connectors that registered jacks use are mainly of the modular connector and 50-pin miniature ribbon connector types. For example, RJ11 uses a six-position two-conductor (6P2C) modular plug and jack, while RJ21 uses a 50-pin miniature ribbon connector.

Naming confusion

The registered jack designation describes a wiring pattern and not just the physical geometry of the connectors; inspection of the connector will not necessarily show which registered jack wiring pattern is used. The same modular connector type can be used for different registered jack connections.

Strictly, registered jack refers to both the female physical connector (modular connector) and its wiring, but the term is often used loosely to refer to modular connectors regardless of wiring or gender, such as in Ethernet over twisted pair. There is much confusion over these connection standards. The six-position plug and jack commonly used for telephone line connections may be used for RJ11, RJ14 or even RJ25, all of which are names of interface standards that use this physical connector. The RJ11 standard dictates a 2-wire connection, while RJ14 uses a 4-wire configuration, and RJ25 uses all six wires. The RJ abbreviations, though, only pertain to the wiring of the jack (hence the name registered jack); it is commonplace but not strictly correct to refer to an unwired plug connector by any of these names.

Modular connectors were a development from the older telephone connectors, which were very bulky or wired directly to the wall and therefore not accommodating of modular systems. The common nomenclature for modular connectors gives the number of contact positions and the number of wires connected, for example 6P indicate a six-position modular plug or jack. A six-position modular plug with conductors in the middle two positions and the other four positions unused is called a 6P2C. RJ11 uses a 6P2C. The connectors could be supplied with more pins, but if more pins are actually wired, the interface is no longer an RJ11.

Twisted pair

Cables terminated in registered jack connectors used in building wiring and the telephone network normally consist of twisted pairs of wires. Wiring conventions were designed to take full advantage of the physical compatibility, thereby ensuring that using a smaller plug in a larger socket would pick up complete pairs, not a (relatively useless) two half pairs. But here again, there has been a problem.

The original concept was that the centre two pins would be one pair, the next two out the second pair, and so on until the outer pins of an eight-pin connector would be the fourth twisted pair. Additionally, signal shielding was optimised by alternating the live (hot) and earthy (ground) pins of each pair. This standard for the eight-pin connector is the USOC-defined pinout, but the outermost pair are then too far apart to meet the electrical requirements of high-speed LAN protocols.

Two variations known as T568A and T568B overcome this by using adjacent pairs of the outer four pins for the third and fourth pairs. For T568A, the inner four pins are wired identically to those in RJ14. In the T568B variant, different pairs are assigned to different pins, so a T568B jack is incompatible with the wiring pattern of RJ14. However, in relatively short connecting cables the performance differences between the pairs that are assigned to different pins are minimal, and T568A and T568B patch cables are usually regarded as interchangeable in general use.

History and authority

Registered jacks were created by industry and regulated by the FCC to be the standard interface between a telephone company and a customer. The wired communications provider (telephone company) is responsible for delivery of services to a minimum point of entry (MPOE) (physically a utility box) which connects the telephone/network wiring on the customer's property (customer-premises equipment/CPE) to the communication provider's network. Customers are responsible for jacks, wiring, and equipment on their side of the MPOE. The intent is to establish a universal standard for wiring and interfaces, and to separate ownership of in-home (or in-office) telephone wiring away from the wiring owned by the telephone company.

Under the Bell System monopoly (following the Communications Act of 1934), the Bell System owned the phones and did not allow interconnection of separate phones or other terminal equipment. Phones were generally hardwired, or at times used proprietary Bell System connectors. This began to change with the case Hush-A-Phone v. United States [1956] and the FCC's Carterfone [1968] decision, which required Bell to allow some interconnection, which culminated in registered jacks.

Registered jacks were introduced by the Bell System under a 1976 FCC order ending the use of protective couplers provided exclusively by the telephone company. The new modular jacks replaced earlier, bulkier connectors. The Bell System issued specifications for the modular connectors and their wiring as Universal Service Order Codes, (USOC), which were the only standards at the time. USOCs are commonly specified to the communications provider by large businesses for a variety of services. Because there are many standardized interface options available to the customer, the customer must specify the type of interface required by RJ/USOC. For a multi-line interface such as the RJ21, the customer must denote which position(s) of the interface are to be used. If there are multiple RJ21 connectors, they are numbered sequentially and the customer must advise the communications provider of which one to use.

When the US telephone industry was opened to more competition in the 1980s, the specifications became US law, ordered by the Federal Communications Commission (FCC) and codified in the Code of Federal Regulations, Title 47 CFR Part 68, Subpart F[1] superseded by T1.TR5-1999.[2]

In January 2001, the FCC delegated responsibility for standardizing connections to the telephone network to a new private industry organization, the Administrative Council for Terminal Attachments[3] (ACTA). The FCC removed Subpart F from the CFR and added Subpart G, which delegates the task to the ACTA. The ACTA generates its recommendations for terminal attachments from the standards published by the engineering committees of the Telecommunications Industry Association (TIA). ACTA and TIA jointly published a standard called TIA/EIA-IS-968[4] which contained the information that was formerly in the CFR.

The current version of that standard, called TIA-968-A,[4] specifies the modular connectors at length, but not the wiring. Instead, TIA-968-A[4] incorporates a standard called T1.TR5-1999, "Network and Customer Installation Interface Connector Wiring Configuration Catalog",[2] by reference to specify the wiring. With the publication of TIA-968-B,[3] the connector descriptions have been moved to TIA-1096-A.[3] Note that a registered jack name such as RJ11 identifies both the physical connectors and the wiring (pinout) of it (see above).

International use

The modular jack was chosen as a candidate for ISDN systems. In order to be considered, the connector system had to be defined under international standards. In turn this led to ISO 8877. Under the rules of the IEEE 802 standards project, international standards are to be preferred over national standards so the modular connector was chosen for IEEE 802.3i-1990, the original 10BASE-T twisted-pair wiring version of Ethernet.

Registered jack types

Common types

The most familiar registered jack is probably the RJ11. This is a modular connector wired for one plain old telephone service line, using two wires of six available positions, and is found in most homes and offices in most countries of the world for single-line telephones. Essentially all one, two, and three line analog telephones made today (2009) are meant to plug into RJ11, RJ14, or RJ25 jacks, respectively.

RJ14 is similar to RJ11 but is wired for two lines and RJ25 has three lines. RJ61 is a similar registered jack for four lines. The telephone line cord and its plug are more often a RJ11 with only two conductors.

The RJ45(S) jack is rarely used, but the designation RJ45 commonly refers to any 8P8C modular connector.

List of official registered jacks

Code Connector Usage
RJ2MB 50-pin micro ribbon 2–12 telephone lines with make-busy arrangement
RJ11(C/W) 6P2C For one telephone line (6P4C if power on second pair)
RJ12(C/W) 6P6C For one telephone line ahead of the key system
RJ13(C/W) 6P4C For one telephone line behind the key system
RJ14(C/W) 6P4C For two telephone lines (6P6C if power on third pair)
RJ15C 3-pin weatherproof For one telephone line
RJ18(C/W) 6P6C For one telephone line with make-busy arrangement
RJ21X 50-pin micro ribbon For up to 25 lines
RJ25(C/W) 6P6C For three telephone lines
RJ26X 50-pin micro ribbon For multiple data lines, universal
RJ27X 50-pin micro ribbon For multiple data lines, programmed
RJ31X 8P8C Allows an alarm system to seize the telephone line to make an outgoing call during an alarm. Jack is placed closest to network interface than all other equipment. Only 4 conductors are used.
RJ32X 8P8C Like RJ31X, this wiring provides a series tip and ring connection through the connecting block, but is used when the customer premise equipment is connected in series with a single station, such as an automatic dialer.
RJ33X 8P8C This wiring provides a series tip and ring connection of a KTS line ahead of the line circuit because the registered equipment requires CO/PBX ringing and a bridged connection of the A and A1 lead from behind the line circuit. Tip and ring are the only leads opened when the CPE plug is inserted. Typical usage is for customer-provided automatic dialers and call restrictors.
RJ34X 8P8C Similar to RJ33X, but all leads are connected behind the line circuit.
RJ35X 8P8C This arrangement provides a series tip and ring connection to whatever line has been selected in a key telephone set plus a bridged A and A1 lead.
RJ38X 8P4C Similar to RJ31X, with continuity circuit. If the plug is disconnected from the jack shorting bars allows the phone circuit to continue to the site phones. Only 4 conductors are used.
RJ41S 8P8C, keyed For one data line, universal (fixed loop loss and programmed)
RJ45S 8P8C, keyed For one data line, with programming resistor
RJ48C 8P4C For four-wire data line (DSX-1)
RJ48S 8P4C, keyed For four-wire data line (DDS)
RJ48X 8P4C with shorting bar For four-wire data line (DS1)
RJ49C 8P8C For ISDN BRI via NT1
RJ61X 8P8C For four telephone lines
RJ71C 50-pin micro ribbon 12 line series connection using 50-pin connector (with bridging adapter) ahead of customer equipment. Mostly used for call sequencer equipment.

Many of the basic names have suffixes that indicate subtypes:

  • C: flush-mount or surface mount
  • F: flex-mount
  • W: wall-mount
  • L: lamp-mount
  • S: single-line
  • M: multi-line
  • X: complex jack

For example, RJ11 comes in two forms: RJ11W is a jack from which you can hang a wall telephone, while RJ11C is a jack designed to have a cord plugged into it. (You can plug a cord into an RJ11W as well, but it usually does not look as nice as a cord plugged into an RJ11C.)

Unofficial plug names

These RJ names do not refer to official ACTA RJ types:

  • RJ9, RJ10, RJ22: 4P4C or 4P2C, for telephone handsets. Since telephone handsets do not connect directly to the public network, they have no registered jack code.
  • RJ45: 8P8C, informal designation for T568A/T568B, including Ethernet; not the same as the true RJ45/RJ45S
  • RJ50: 10P10C, often used for data

RJ11, RJ14, RJ25 wiring details

All of these registered jacks are described as containing a number of potential contact positions and the actual number of contacts installed within these positions. RJ11, RJ14, and RJ25 all use the same six-position modular connector, thus are physically identical except for the different number of contacts (two, four and six respectively) allowing connections for 1, 2 or 3 phone lines respectively.

RJ11 cables should then need to use only 6P2C connectors. Nevertheless, often cables sold as RJ11 still use 6P4C (RJ14) connectors (six position, four conductor), with four wires running to a central junction box. Two of the six possible contact positions connect tip and ring, and the other two conductors are then unused. 6P2C and 6P6C can also be found in stores.

The conductors other than the two central tip and ring conductors are in practice used for various things such as a second or third phone line, a ground for selective ringers, low voltage power for a dial light, or for 'anti-tinkle' circuitry to prevent pulse dialing phones from ringing the bell on other extensions. With tone dialing anti-tinkle measures are not required.


Holding the connector in your hand tab side down with the cable opening toward you, the pins are numbered 1–6, left to right.

Position Pair T/R ± RJ11 RJ14 RJ25 25-pair color code U.S. Bell System colors[a] German colors[b] Australian colors
1 3 T + T3



2 2 T + T2 T2



3 1 R R1 R1 R1



4 1 T + T1 T1 T1



5 2 R R2 R2



6 3 R R3



  • ^[a] While the old solid color code was well established for pair 1 and usually pair 2, there are several conflicting conventions for pair 3 (and sometimes even pair 2). The colors shown above were taken from a vendor of "silver satin" flat 8-conductor phone cable that claims to be standard. 6-pair solid (old) bellwire cables previously used by the Bell System use white for pair 3 tip but some vendor's cable may substitute orange for white. At least one other vendor of flat 8-conductor cable uses the sequence blue, orange, black, red, green, yellow, brown and white/slate.
  • ^[b] This color scheme originates in the (withdrawn) national standard DIN 47100. The scheme shown here is the correct color code for interfacing with the RJ connector standards.

However, with German domestic telephone equipment (and that in some neighbouring countries), 6P4C plugs and sockets are typically only used to connect the telephone cable to the phone base unit, whereas the mechanically different TAE plug is used at the other end of the cable. Older base units may accommodate the additional connectors of TAE (E, W, a2, b2) and may feature non-RJ standard sockets that can be connected „straight“ to TAE plugs. Further, flat DIN 47100 cables typically place the wires in ascending order. When used directly with 6P4C plugs, the colors will be garbled.

Powered version of RJ11

In the powered version, Pins 2 and 5 (black and yellow) may carry low voltage AC or DC power. While the phone line itself (tip and ring) supplies enough power for most telephone terminals, old telephone instruments with incandescent dial lights in them (such as the classic Western Electric Princess and Trimline telephone models) needed different voltages than the phone line supplies. Typically, the power on Pins 2 and 5 came from a transformer plugged into a power outlet near one jack, wired to supply power to just that telephone (or to all of the jacks in the house, depending on local telephone company practices). It is now usually recommended that only the one local jack used by such older telephones be wired for power, to avoid any potential interference with other types of service that might be using pins 2 and 5 (black and yellow pair) in jacks in other parts of the house. The early Trimline and Princess phone dial light incandescent lamps were rated at 6.3 Volts, 0.25 Amp and the transformer output is approximately 6-8 Volts AC (while later Trimline versions had LED light sources, powered directly from the phone line, and the last Western Electric-made Princess version had no dial light).


RJ21 is a registered jack standard for a modular connector using 50 conductors, usually used to implement a 25-line (or less) telephone connection such as that used in the 1A2 key telephone system. It is also known as a 50-pin telco connector, CHAMP(AMP) or an Amphenol connector (the latter is a genericized trademark, as Amphenol was the largest manufacturer of these at one time).

Dual RJ21 connectors are often used on punch blocks to make a breakout box for PBX and other key telephone systems.

RJ21 connectors are used to connect Ethernet ports in bulk from a switch with RJ21 ports to a CAT-5 rated patch panel, or between two patch panels. A cable with an RJ21 connector on one end can support 6 fully wired 8P8C connectors or Ethernet ports on a patch panel with 1 spare pair. Alternatively, only the necessary pairs for 10/100 Ethernet can be wired allowing 12 Ethernet ports with a single spare pair.

The same connector (with spring bail locks) is used for SCSI-1 connections. Some computer printers use a shorter 36-pin version known as a Centronics connector.

A cable color scheme, in common use, is determined for 25 pairs of conductors as follows:[5] For each ring, the primary, more prominent color is chosen from {blue, orange, green, brown, slate}, in that order, and the secondary, thinner stripe color from {white, red, black, yellow, violet}, in that order. The tip conductor color scheme uses the same colors as the matching ring but switches the thickness of the primary and secondary colored stripes. Since the sets are ordered, an orange (color 2 in its set) with a yellow (color 4) is the color scheme for the 4·5 + 2 − 5 = 17th pair of wires. If the yellow is the more prominent, thicker stripe, then the wire is a tip conductor connecting to the pin numbered 25 + the pair #, which is pin 42 in this case. (Ring conductors connect to the same pin number as the pair number.)

A conventional enumeration of wire color pairs then begins blue (& white), orange (& white), green (& white) and brown (& white), which subsumes a color-coding convention used in cables of 4 or fewer pairs (8 wires or less) with 8P and 6P connectors.

Color Pin (Tip) Pin (Ring) Color
White/Blue 26 1 Blue/White
White/Orange 27 2 Orange/White
White/Green 28 3 Green/White
White/Brown 29 4 Brown/White
White/Slate 30 5 Slate/White
Red/Blue 31 6 Blue/Red
Red/Orange 32 7 Orange/Red
Red/Green 33 8 Green/Red
Red/Brown 34 9 Brown/Red
Red/Slate 35 10 Slate/Red
Black/Blue 36 11 Blue/Black
Black/Orange 37 12 Orange/Black
Black/Green 38 13 Green/Black
Black/Brown 39 14 Brown/Black
Black/Slate 40 15 Slate/Black
Yellow/Blue 41 16 Blue/Yellow
Yellow/Orange 42 17 Orange/Yellow
Yellow/Green 43 18 Green/Yellow
Yellow/Brown 44 19 Brown/Yellow
Yellow/Slate 45 20 Slate/Yellow
Violet/Blue 46 21 Blue/Violet
Violet/Orange 47 22 Orange/Violet
Violet/Green 48 23 Green/Violet
Violet/Brown 49 24 Brown/Violet
Violet/Slate 50 25 Slate/Violet


The RJ45 physical connector is standardised as the IEC 60603-7 8P8C modular connector with different "categories" of performance, with all eight conductors present. A similar standard jack once used for modem/data connections, the RJ45S, used a "keyed" variety of the 8P8C body with an extra tab that prevents it mating with other connectors; the visual difference compared to the more common 8P8C is subtle, but it is a different connector. The original RJ45S [6][7] keyed 8P2C modular connector had pins 5 and 4 wired for tip and ring of a single telephone line and pins 7 and 8 shorting a programming resistor, but is obsolete today.

Electronics catalogs commonly advertise 8P8C modular connectors as "RJ45". An installer can wire the jack to any pin-out or use it as part of a generic structured cabling system such as ISO/IEC 15018 or ISO/IEC 11801 using 8P8C patch panels for both phone and data. Virtually all electronic equipment which uses an 8P8C connector (or possibly any 8P connector at all) will document it as an "RJ45" connector.


RJ48 is a registered jack. It is used for T1 and ISDN termination and local area data channels/subrate digital services. It uses the 8 position modular connector.

RJ48C is commonly used for T1 lines and uses pins 1, 2, 4 and 5.

RJ48X is a variation of RJ48C that contains shorting blocks in the jack so that a loopback is created for troubleshooting when unplugged by connecting pins 1 and 4, and 2 and 5. Sometimes this is referred to as a "smart jack".

RJ48S is typically used for local area data channels/subrate digital services, and carries one or two lines. It uses a keyed variety of the 8P8C modular connector.

RJ48 connectors are fastened to STP (shielded twisted pair) cables, not the standard UTP (unshielded twisted pair) CAT-(1-5).

RJ48C and RJ48X wiring
Pin Pair Signal Color
1 R RX Ring Orange/White
2 T RX Tip White/Orange
3 reserved White/Green
4 R1 TX Ring Blue/White
5 T1 TX Tip White/Blue
6 reserved Green/White
7 shield White/Brown
8 shield Brown/White


RJ61 is a physical interface often used for terminating twisted pair type cables. It is one of the Registered Jacks, and uses an eight position, eight conductor (8P8C) modular connector.

RJ61 specifies the wiring pinout of the connector as follows:

RJ61 wiring (USOC)
Pin Pair Wire Color
1 4 tip White/Brown
2 3 tip White/Green
3 2 tip White/Orange
4 1 ring Blue/White
5 1 tip White/Blue
6 2 ring Orange/White
7 3 ring Green/White
8 4 ring Brown/White

This pinout is for multi-line telephone use only; RJ61 is unsuitable for use with high-speed data, because the pins for pairs 3 and 4 are too widely spaced for high signaling frequencies. T1 lines use another wiring for the same connector, designated RJ48. Ethernet over twisted pair (10BASE-T, 100BASE-TX and 1000BASE-T) also use a different wiring for the same connector, either T568A or T568B. RJ48, T568A, and T568B are all designed to keep pins close together for pairs 3 and 4.

The flat 8-conductor "silver satin" cable traditionally used with 4-line analog telephones and RJ61 jacks is also unsuitable for use with high-speed data. Twisted pair cabling must be used with RJ48, T568A and T568B. Note that data twisted pair patch cable used with the three data standards above is not a direct replacement for RJ61 cable, because RJ61 pairs 3 and 4 would be split among different patch cable twisted pairs, causing cross-talk between voice lines 3 and 4 that might be noticeable for long patch cables.

With the advent of structured wiring systems and TIA/EIA-568-B conventions, the RJ61 pinout is falling into disuse. The T568A and T568B standards are used in place of RJ61 so that a single wiring standard in a facility can be used for both voice and data.

See also

References and notes

Further reading

  • RJ glossary
  • RJ48C, RJ48S, RJ48X
  • T1 & T3 RJ48 cables
  • Registered Jack reference guide
  • Administrative Council for Terminal Attachments, section 6.2 in particular
  • ANSI/TIA-1096-A
  • UTP cable termination standards 568A vs 568B
  • John R. Carlsen: On wiring modular telephone connectors
  • Modular jack wiring
  • Modular wiring reference showing differences between 8P8C, true RJ45 8-position keyed connector, 6P6C, and 6-position modified offset tab
  • Common outlet configurations graphical representation of twisted pair pinouts

External links

  • Administrative Council for Terminal Attachments
  • Doing your own telephone wiring
  • Connecting a second phone line
  • How to crimp wire impact an RJ45 jack Impact tool
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