An electrician has two two-way switches (single-pole, double-throw), a light bulb, and a power source. How should he connect the terminals so that either switch can be used to turn the light on or off?
The terms pole and throw are used to describe switch contact variations. A pole is a set of contacts or terminals that are connected to a single circuit. A throw is one of two or more positions that the switch can adopt. A single-throw switch has one position that closes contacts, a double-throw switch has two positions.
"Single-pole, double-throw" switches, such as the ones illustrated in this puzzle, are called "two-way" switches in British English, whereas they are generally called "three-way" switches in American terminology. A "two-way" switch in American English refers to a "single pole, single throw" switch.
Terminal D must connect to terminal I, and terminal E must connect to terminal J. D to J and E to I will also work. The light bulb and the power source must be connected in series, A to G, for example. Terminal F then connects to C and terminal B connects to H. F to H and B to C will also work. Either switch can then turn the bulb on or off.