2 light switches controlling one light fixture?

It's common for less experienced handypeople to get tripped up on 3-ways (and switched receptacles). Here is how 3-way switches work. (Called 2-way in the UK).

Obviously, your wires are not yellow. In fact, wiring color coding isn't very strong at all, since the color of wires is decided by the cable manufacturer, not by the usage (wouldn't that be nice).

But what you notice is each switch has 3 wires, and two of them are travelers. The third one is called a "common" on the switch, and can have one of two tasks, as you see in the diagram.

The usual mistake made by novices is mixing the wires up. Typicaly they are marked on the switch as screw color or legend, but people don't think to look for that, they often freestyle their own way of matching wires. And it often doesn't work.

So when you are confronted with a situation where that information was lost, what do you do?

First, you know that both travelers run together, so they must both be in the same cable on both ends. So look closely at the wiring to one of them. Follow the wires back to which cables they come from. If two of them are in the same cable, those are your travelers. If all 3 are in the same cable, you can't tell: check the other switch.

We also know colors don't change mid-cable. So if the travelers are black and red at one switch, most likely it is black and red on the other switch (unless there is an intermediate splice somewhere).

Worse comes to worst, there are only 3 possibilities, since travelers are swappable. Just try one at a time until it works.

My policy is that when I know which ones are travelers, I mark them both with colored tape, (not black or brown), same color since travelers are interchangeable and there's no need to tell them apart.

Do Not Unhook Any Other Wires. You'd be amazed how many photos we get of six neatly cut and stripped wires and "what do I do now?" When they were only changing a 1-way switch...