BJT H-Bridge.

Suppose you needed for current to be able to flow two directions through a component. Then you need 4
 switches around the component to form an H-bridge. You can do this with 4 transistors as well, 2 NPN or 2 PNP. You could also use FET's in a similar arrangement.

I recently realized that you could control both switches in the pair if you hooked it up right. Play with it on the Circuit Simulator.

I plan on using this arrangement to drive a two way latching a relay. A Brief pulse on one side sets the relay, and a brief pulse on the other side resets it.