The diagram shows the common method of wiring an intermediate switch into a two way light switch circuit. Staircase wiring using intermediate switch.

Intermediate Switch Wiring Diagram Pdl C3 Wiring Diagram
Wiring a intermediate light switch. Four way switch wiring instructions inside the switch packaging. Because the electrical code as of the 2011 nec update requires a neutral wire in most new switch boxes a 3 wire cable runs between the light and switch. This is an updated version of the first arrangement. This allows a single light to be turned on or off from any of the switches. Here we have a 3 way switching lighting circuit sometimes called two way switching with intermediate. An intermediate switch is a three way light switch.
It is used when you have three or more switches controlling one light the middle switch needs to be an intermediate light switch. Intermediate switches can be used for 1 and 2 way switching. It is effectively two single gang intermediate switches on one face plate. Below is a given schematic circuit to control a light point from three different places by using two 2 way switches and intermediate switch in the middle of stairs. The red and black are used for hot and the white neutral wire at the switch box allows for powering a timer remote control or other programmable switch. It is possible to bring all of the wiring into the ceiling light electrical box and wire your 3 ways and 4.
Sometimes there is only a switch wiring diagram printed on the inside of the 4 way switch package box. The intermediate light switch is used where three or more switches control one light and used in conjunction with two two way light switches to achieve this. The wiring connections on 4 way switches varies between manufacturers. If replacing an existing switch note the location of the terminals and the colour and position of cable connections. The typical example of this is on a landing in a house when perhaps there is a switch at the bottom of the stairs one at the top of the stairs and one at the end. This page shows it wired in the old cable colours if your house has the new colours you want to go here.