Switch Terminology
There seems to be some controversy in switch terminology on a single on/off switch and how it's referred too. Some people call them 2-way and some call them SPST or toggle switch. SPST stands for "single-pole-single-throw" which means the switch has 1 common (single-pole) and 1 out (single-throw). Now on this site, I will be referring to switches as 2-way, 3-way etc..
The switch terminology reflects on how each type of switch functions. A 2-way switch simply means you are controlling 1 wire in & 1 wire out, SPST (single-pole-single-throw).
A 3-way switch is 1 wire in & 2 wires out or SPDT (single-pole-double-throw).
A 4-way switch is 2 wires in & 2 wires out or DPDT. You guessed it (double-pole-double-throw).
Let's look at the diagram below:
It clearly shows how these combination of switches operate. The common is usually referred to as the contact that always has one end of the switch attached. In this case it is the contacts on the left side of the switches. When the switch is toggled, the opposing contacts are connected or disconnected which opens and closes the circuit.
To better understand the pole and throw theory, Let's look at it like this..Look at the left side of the switches which is the common side. A pole is how many seperate circuits are being used. The 2-way has 1 circuit, the 3-way has 1 circuit and the 4-way has 2 circuits. Now for the throw side (right side of switch contacts). The 2-way has one circuit, The 3-way has 2 circuits and the 4-way has 2 circuits.
Now the 3-way switch is a little different. When toggled, the switch is just alternating between the 2 circuits. When the switch is in the up position, the upper circuit is connected (closed) and the lower circuit is disconnected (open). When the switch is in the down position then you have just the opposite. The 4-way is doing the same thing but it is alternating the 2 incoming poles to either of the 2 output poles. When the top common pole is connected to the bottom output pole then the bottom common pole is connected to the upper output pole. Clear as mud right.
