Look at any Ethernet cable, RS-485 run, 4-20 mA sensor wire, or telephone line and you'll find the same construction: pairs of insulated conductors twisted together at a specific pitch. That twist isn't cosmetic — it's a physics trick that lets low-voltage signals survive a factory floor full of VFDs, motor starters, and 480 V bus ducts.

The problem: differential signals and common-mode noise. A twisted pair carries a signal as the difference between two conductors. The receiver ignores whatever voltage both wires share. Noise from a nearby motor or fluorescent lamp couples electromagnetically into both wires — but only if the wires present different loop areas to the field. If one wire happens to run closer to the noise source than the other, it picks up more, the difference between them changes, and the noise becomes signal.

The fix: twist them. Each half-twist swaps which wire is on the "near" side of any external field. Over a length of cable, the induced voltages average out to nearly zero on both conductors — same on both means common-mode, which the receiver rejects. The tighter the twist (shorter the lay length), the better the noise cancellation and the higher the usable frequency.

Cat cable examples:

• Cat5e: ~1.5-2 twists per inch on some pairs, up to ~4 on others, rated to 100 MHz
• Cat6: tighter twists plus an internal plastic spline separating pairs, rated to 250 MHz
• Cat6a/Cat7: even tighter twists plus foil shields per pair for 10 Gbps at 500-600 MHz

Different pairs in the same jacket use different lay lengths on purpose — if all four pairs twisted at the same pitch they'd couple into each other (crosstalk) in sync.

Rule of thumb for installers: when terminating a jack or a punch-down block, do not untwist more than ½ inch (13 mm) of pair. The last inch of an untwisted pair is where noise sneaks in and where impedance discontinuities cause reflections. Field techs who "fan out" three inches to make terminations pretty are the reason a certified Cat6 run fails its NEXT (near-end crosstalk) test.

Instrumentation twisted pair (belden-style) uses much longer lay lengths (~2-3 inches) because the signals are DC or low-frequency 4-20 mA, but the same principle applies: run the pair, don't split it, and if you need shielding for really nasty environments, add a foil or braid and ground one end only to prevent ground loops.