Staircases are generally equipped with a light bulb that can be operated from either of the two switches that are located at its two ends. The idea is simple – you should be able to switch on the bulb from one end and then switch it off after you have climbed up/down the staircase from the other – and the configuration is such that if you toggle either of the two switches from their current state (on/off) you change the state of the bulb (on/off).

Have you ever wondered how can you build such a circuit using a bulb and two switches? Grab a pen and a piece of paper and try drawing a circuit diagram. Here’s a circuit diagram for a bulb connected to a battery using a switch.

To know more select the part between the two asterix signs.

*         The switches used for staircase lights are actually three-way switches. They look the same from outside as the regular two-way switches; the difference being that while two-way switches toggle between disconnecting a circuit (off position) and connecting it (on position), the three-way switch toggles between connecting one of the ends (say A) to one of the other ends (say B in off position, C in on position) *

Here is a very simple and intuitive circuit diagram to explain it further.