Just moved house. Lounge light has two switches at either end of room. One switch (say A)works light other (B)doesn't.
Switch B only has red & black wire at switch. Switch A is a double switch which also works landing light. This switch has has red, black (for landing) red & black (for lounge)- plus the other end of red & black from Switch B, plus one yellow
I don't see how this would ever work without a third yellow wire to Switch B as Common?
Any ideas please?

heeelllooo and welcome crephi

each light switch is only connected to its own light/s[ and other switches connected to the same light

what you see is the live wire comming to the switch and returning via the other wire [that should be red sleeved] back to the light
the power goes from the rose to the bulb on the neutral side and from the rose to the bulb via the switch on the live side

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we are all ------------------still learning