6376. Each one acts as a destination point for a teleportation system, and gets a unique address when you build it (don't think you can have more than one in the same chunk). You then dial the address for another gate from your gate, and it creates a link so you step through one gate and out the other. I believe it can also work across dimensions, but requires a 9-digit address rather than the usual 7, and thus the gate needs to be upgraded