An inner track on the Stargate moves and halts in a particular place, allowing a chevron to snap down and back up, storing the cymbol in its memory until all the chevrons have locked. Ordinarily seven chevrons are required to lock on to a destination.
Six chevrons determine a specific point in space. The seventh chevron applies as the point of origin, allowing two points in space to connect and generate an outgoing wormhole.
A more uncommon eighth chevron is believed to be the equivalent to dialing, in essence, a diffrent area code. Evidence proves that the eighth chevron dials beyond normal space into an entirly diffrent galaxy, but this typically causes extreme power consumption.
The purpose of the ninth chevron is, as yet, unclear.
Stargate Technology Index