Single switches have short thin leads that don't stay on the breadboard. All other sizes have longer, stiffer wires and work well. I've attached a photo of a single switch next to a double switch. The little box they come in is disorganized. It comes with 9 compartments, but some compartments do not fit all circuit breakers of the same size, so the breakers are either placed in other compartments or in the 9th compartment, which contains one of three different sizes.
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