These diagrams show various methods of one, two and multiple way switching.
L and N indicate the supply. Switches are shown as dotted rectangles. Earth wires are not shown.
It can also be used in domestic properties by using twin+earth cable between the switches, and 1core+earth from the switches to the ceiling rose.
Unfortunately, this is usually encounted in stairwells, with the line from the downstairs lighting circuit and the neutral connected to the upstairs lighting circuit. Such an arrangement is not permitted, as isolating only one of the circuits leaves live wiring depending on the position of the light switches.
This is also known as the 'conversion' method, since it is the easiest way to add a second light switch to an existing circuit.
Both of the three way switching diagrams can be extended to four, five or even more switches. All of the additional switches are internediate types (4 terminals), and connect into the middle of the circuit in exactly the same way.
A dimmer switch can be used on any of these circuits, but for two switches and above, only one of them can be a dimmer. The single dimmer sets the light level, and the other normal switches just switch the light off and on at whatever level the dimmer is set to.