Two-way and intermediate switchingHow do you switch one light from three places?

Two-way switching

A two-way switch has 3 terminals: COM (common), L1 and L2. Pressing it doesn't turn it "on" or "off". It just swaps COM between L1 and L2. The two switches are linked by 3-core & earth cable (brown, black, grey plus a bare CPC). The two cores that link L1 to L1 and L2 to L2 are the strappers; the third core carries the common.

• Sleeve every strapper brown (black and grey), because they're all line conductors. You could buy all-brown 3-core, but then you couldn't tell which is which.
• There's no right or wrong colour for COM, L1 or L2. Swap the two strappers (L1 and L2) at one end and it still works. What stops it working is mixing COM with a strapper, so keep COM to COM.
• Coiling the CPC neatly in the back box is purely cosmetic. It adds length, and length means resistance.

On-Site Guide · Section 7 · Figure 7.4.4 two-way switching wiring (black in COM in that drawing) · p.107

Intermediate switching

To control a light from 3 or more places, fit intermediate switches between the two two-way switches. You can have as many as you need, e.g. a stairwell over several floors or a big room with several doors. They're mainly commercial, and rare in homes.

Wire it as before, common to common. The common conductor (whatever colour you used in COM) doesn't connect to the intermediate. It just joins through in a connector in the back box. It isn't an earth.