Switches are generally named after the number of poles and throws. The leading letter (or digit) stands for the number of poles, the third letter for the number of throws. (S for Single, D for Double, T for Triple, Q for Quadruple). A pole always connects to one or more throws.

SPDT and SPST — Single Pole

The number of poles denotes how many separate internal switches are controlled by toggling the big switch. The number of throws denotes how many wiring path choices the switch can adopt for each pole. Here’s a Single Pole, Double Throw switch:

[SPDT on-off-on: internal wiring schematic]

[SPDT on-off-on: internal wiring schematic]

The middle contact (also known as COM for Common) is connected to either the contact above or below. If the switch has three positions, the middle position will leave COM disconnected from a and b.

Double-throw switches can be turned into single-throws by simply ignoring one of the outer lugs so that the circuit is either closed (left) or open (right):

[SPST: internal wiring schematic]

[SPST: internal wiring schematic]

DPDT and 3PDT — Multiple Poles

A switch with more than one pole has multiple separate, parallel sets of contacts that change simultaneously when the switch is toggled. Equivalent to the SPDT, the COM lugs are connected to either those above, below, or neither.

[DPDT on-off-on: internal wiring schematic]

[DPDT on-off-on: internal wiring schematic]

The orientation of double-throw switches usually matches the schematics shown here when you can thread a needle through all lugs from top to bottom. DPDT and 3PDT switches are popular choices for True Bypass Wiring.

[3PDT on-on: internal wiring schematic]

[3PDT on-on: internal wiring schematic]

DPDT Type 1 vs. Type 2

Other than on-off-on DPDTs, there are on-on-on versions as well. These come in Type 1 and Type 2 configurations. In the middle position, one COM lug switches to the lug below, while the other stays connected to the one above.

[DPDT on-on-on, Type 1]

[DPDT on-on-on, Type 1]

[DPDT on-on-on, Type 2]

[DPDT on-on-on, Type 2]

SP3T/DP3T – Triple Throw Switches

Some triple-throw switches don’t actually have a third throw, but manufacturers refer to the middle position as such. The SP3T looks a lot like a DPDT except that the datasheet tells you to connect the two lugs as shown with the black wire.

[SP3T on-on-on: wiring schematic]

[SP3T on-on-on: wiring schematic]

So what’s sold here is actually a DPDT Type 2 switch that can be modded into an SP3T. The same can be done easily with a 4PDT on-on-on to turn it into a DP3T. Always look at the datasheet before buying a switch.