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My favorite implementation: the Knife Switch!

Everyone knows about those huge switches used by mad scientist movies to transfer the electricity of lightning to their latest creation.

Smaller versions available at Radioshack cannot (unfortunately) handle lightning.

Ascii art diagram:

   a   X   a

   b   X   b

Each letter is a connection node, it's basically two AB Switches built into one (with a neutral, open circuit stage between A and B.) I used knife switches to build my house's intercom system using existing phone wiring in the house.

Very convenient for rerouting the entire circuit to a different device!