Two-way switching
Controlling one light from two switches — flick either one to change the state.
What it does
Two-way switching lets you turn one light on or off from two separate places — top and bottom of stairs, both ends of a hallway, two doors of a room.
How it works
Each switch is a changeover (it has a common terminal and two others). The two switches are joined by a pair of conductors called travellers. The supply feeds the common of one switch; the lamp connects to the common of the other.
Whichever way the switches are set, the light is on only when the travellers complete a path between the two commons. Flicking either switch changes which traveller is connected, making or breaking that path — so either switch toggles the light regardless of the other's position.
Extending it
Adding an intermediate switch between the two travellers lets you control the same light from three or more positions. The travellers are the key idea — get those right and the logic follows.
When it misbehaves — and how to prove it
The tell-tale symptom: the light works from one switch only, or only with the other switch in one particular position. That's crossed conductors — the common on a traveller terminal, almost always after a switch or plate swap. A completely dead light is usually just a lighting fault (lamp, supply), not the switching.
Prove it dead with continuity: at each switch, find the true common — it's the terminal that connects to one traveller in one switch position and the other traveller in the other. Then confirm the feed lands on switch one's common and the switch wire to the lamp on switch two's common, with the travellers linking the rest. Ten minutes with a meter beats an afternoon of swap-and-hope.
Safety first
Isolate and prove dead before working — at a switch you may have an unswitched live, a switched live, and travellers all present. Don't assume which is which.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
A single light not working
One light fitting is dead while the rest of the lights on the circuit work fine — points at the lamp, the fitting, or the switch for that point rather than the whole circuit.
LED lights glowing faintly when switched off
LED lamps glow dimly or pulse even after the switch is off — unsettling but usually a small leakage/induced-voltage effect rather than a dangerous fault.
Smart switch not working (no neutral at the switch)
A smart/Wi-Fi switch won't power up, drops offline, or makes the light glow/flicker — commonly because there's no neutral at the switch, which many smart switches need.