US What T568A and T568B are
T568A and T568B are the two wire color codes for terminating twisted-pair Ethernet cable on an eight-position RJ45 (8P8C) plug, jack or patch panel. Both come from the ANSI/TIA-568 commercial cabling standard, and the international ISO/IEC 11801 and European EN 50173 standards use the same pin-to-pair assignments.
An Ethernet cable holds four twisted pairs, colored blue, orange, green and brown, and each pair has a solid wire and a white wire striped in the same color. A wiring scheme simply says which wire lands on which pin. T568A and T568B agree on the blue and brown pairs and differ only on the orange and green pairs, which trade places on pins 1, 2, 3 and 6.
Neither scheme is faster or more reliable. The pins carry the same signals either way, so a link wired T568A at both ends works exactly like one wired T568B at both ends. The color code matters for consistency, not for performance.
RJ45 color code and pinout chart
The chart reads pins 1 to 8 from left to right with the plug’s clip facing away from you and the contacts on top. The last two columns show what each pin carries on Fast Ethernet and Gigabit Ethernet ports, following IEEE 802.3.
| Pin | T568A color | T568B color | 10/100BASE-TX | 1000BASE-T |
|---|---|---|---|---|
| 1 | White/green | White/orange | TX+ | BI_DA+ |
| 2 | Green | Orange | TX− | BI_DA− |
| 3 | White/orange | White/green | RX+ | BI_DB+ |
| 4 | Blue | Blue | Not used | BI_DC+ |
| 5 | White/blue | White/blue | Not used | BI_DC− |
| 6 | Orange | Green | RX− | BI_DB− |
| 7 | White/brown | White/brown | Not used | BI_DD+ |
| 8 | Brown | Brown | Not used | BI_DD− |
Fast Ethernet (10BASE-T and 100BASE-TX) uses only the pairs on pins 1–2 and 3–6, which are exactly the pairs the two schemes swap. Gigabit, 2.5G, 5G and 10GBASE-T use all four pairs in both directions at once, so every pin is live. That is why a cable with a damaged blue or brown pair can still pass 100 Mb/s but not Gigabit.
Why there are two wiring standards
T568A was the scheme TIA preferred when the standard was first written, because its pair order is backward compatible with the older USOC telephone wiring on the centre pins and with both one-pair and two-pair phone circuits. US federal government contracts historically called for T568A for the same reason.
T568B matches the older AT&T 258A color code, which was already widely installed in commercial buildings when TIA-568 was published, so the standard allowed it as an alternative rather than force a rewire. Most pre-made patch leads and a large share of installed cabling now use T568B, both in the US and the USA. Current revisions of the standard accept either, as long as one scheme is used throughout an installation.
Straight-through vs crossover cables
A straight-through cable uses the same scheme at both ends, so pin 1 connects to pin 1 and so on. It is the normal cable for connecting a computer, camera or panel PC to a switch, and for every run in a structured cabling system.
A crossover cable is wired T568A at one end and T568B at the other, which swaps the transmit and receive pairs. It was once needed to connect two computers or two switches directly. Today almost every Gigabit port supports Auto MDI-X, which detects the pair arrangement and swaps it internally, so a straight-through cable works in either case. Crossover cables are now mostly kept for old 10/100 equipment. A true Gigabit crossover swaps all four pairs, not just orange and green, which a simple A-to-B cable does not do.
T568A vs T568B in industrial networks
Industrial sites add a few points that office cabling rarely meets. None of them changes which scheme you choose, but each depends on keeping the scheme consistent.
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Power over Ethernet. PoE Mode A carries power on the pairs on pins 1–2 and 3–6, and Mode B on pins 4–5 and 7–8. Because T568A and T568B only swap pairs between those same pins, PoE works with either scheme. Our guide to industrial Power over Ethernet covers the modes, classes and power budgets.
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Field-terminated plugs. On machines and in cabinets, cables are often cut to length and terminated on site. A wrongly ordered plug, such as one with a split pair, often still links at 100 Mb/s because only two pairs are in use, then fails or logs errors at Gigabit, which makes the fault look intermittent.
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M12 to RJ45 cables. Rail, vehicle and machine networks use circular M12 connectors on the device and RJ45 at the cabinet. D-coded M12 carries two pairs for 100 Mb/s and X-coded M12 carries four pairs for up to 10 Gb/s, and the RJ45 end is terminated to T568A or T568B like any other plug. M12 Ethernet switches take these cables directly.
Which one should you use?
Use whichever scheme the site already uses. If you are starting from nothing, T568B is the common choice because it matches most pre-made patch leads and most installers’ habits. Choose T568A if a contract or a customer’s cabling specification calls for it, which is still common on some government and older telecom installations.
Whichever you choose, wire every jack, patch panel and plug the same way and record it in the cabling documentation. Runs wired to different schemes still work, as long as each run uses the same scheme at both ends. Mixing them makes faults much harder to trace, though, and a run with an A jack at one end and a B jack at the other is the usual cause of an accidental crossover in a permanent link.
Cable category is a separate choice. The color code is the same for Cat5e, Cat6 and Cat6A, so check our comparison of Cat5e vs Cat6 for speed and distance, and our explainer on what RJ45 is for the connector itself.
Quick answers on T568A vs T568B
T568A and T568B: common questions
Is T568A or T568B more common?
T568B, in most places. Most pre-made patch leads and most commercial installations in the US and the USA use T568B. T568A remains common in homes in some regions and where a contract specifies it.
What happens if you mix T568A and T568B?
You get a crossover cable. A cable wired A at one end and B at the other swaps the orange and green pairs. Modern ports with Auto MDI-X correct for it and usually link normally, but inside a fixed installation it breaks the documented wiring and confuses later fault-finding.
How can you tell if a cable is T568A or T568B?
Look at pin 1 through the clear plug. Hold the plug with the clip facing away and the contacts on top. If the leftmost wire is white/orange, it is T568B. If it is white/green, it is T568A.
Is T568B faster than T568A?
No. Both schemes use the same pins and the same pairs, so they support the same speeds, from 10 Mb/s up to 10GBASE-T on suitable cable. Performance depends on the cable category, the termination quality and the length.
In T568B, what color is pin 6?
Green. Pin 3 is white/green and pin 6 is solid green, which together form the second data pair used by 10/100 Mb/s Ethernet.
Industrial Ethernet works the same way at the edge of the network: our industrial Ethernet switches are supplied with the computers and panel PCs on a project, and we cable, configure and test them together as standard before the system ships.