If you are searching online for 2 way switches/switching and 3 way stitches/switching the results can be very confusing to the layman depending on wether you land on a US or UK website. This is because the terminology used is very different.
This site follows UK usage. So for the benefit UK visitors searching online and American visitors to this site, hopefully the following will help to clarify:
UK – So, this (Fig 1) is what we call a 2 way switch: when you switch it one ‘way’ (on or position 1) the COM terminal is connected to the L1 terminal. When you switch it the other ‘way’ (off or position 2) the COM terminal connects to the L2 terminal.
If you are describing this switch to an electrician or electrical supplier you should refer to it as a 1 gang 2 way light switch and they will know exactly what you mean (see gangs and ways).
US – Strangely, our friends across the pond refer to this switch as a 3 way switch because it has 3 terminals.
Generic – if you asked someone in the field of electronics, they would probably describe this switch as a Single-pole, Double-Throw (SPDT) switch, to my mind a more useful description as this means the same in UK and US. A good electrician would understand this terminology but they, historically, rarely use it.
UK – so, this (Fig 2) is what we call an intermediate switch, so called because it is used ‘in between’ two x 2 way switches to achieve 3 way switching (clarified below under multiway switching). Sometimes called a crossover switch for obvious reasons.
If you are describing this switch to an electrician or electrical supplier you should refer to it as a 1 gang intermediate light switch and they will know exactly what you mean.
US – in America they refer to this switch as a 4 way switch because it has 4 terminals.
Generic – Double-pole, Double Throw (DPDT) switch.
Now we understand the different terminology used for the actual switches it is easy to appreciate why different switching arrangements are referred to using different terminology also.
Scenario 1: we have 1 light that can be turned on and off from two different light switches:-
UK – we would call this 2 way switching. Why? well a layman probably uses this terminology because 2 switches are used, whereas an electrician because two 2 way switches are used to achieve this. Either way, we are both referring to the same thing.
US – This would be referred to as 3 way switching because its uses what they would refer to a two 3 way switches.
Scenario 2: we have 1 light that can be turned on and off from three different light switches:-
UK – we would call this 3 way switching, well a layman certainly would (because of the three switches) and an electrician would assume this is what is meant but may also refer to it as 2 way plus intermediate switching.
US – This would be referred to as 4 way switching because it utilises what they refer to as a 4 way switch (what we in the UK call an intermediate or crossover).
I’m including this method for reference in case you find it used in your house wiring but would not recommend this approach in a domestic environment. If you are adding wiring for a three way light switch then use the 3 wire control system.
I have detailed, quite extensively, why a two wire control system can be problematic in the article 2 way switch with 2 wire control. All the issues raised there apply here too, especially if used in stairways and the switches are on different floors.
This method is best suited to conduit/trunking based installations where single core conductors are used.