Seventh symbol
From The Stargate Omnipedia
Category:
Seventh symbol
Generally the last symbol in a series of seven, acting as the point of origin. The seventh symbol "locks" the seventh chevron, which generates an active lock with another Stargate, initiating an active wormhole.
Seventh symbols can be located beneath the six other symbols on Stargate cover stones, and can be mistaken as part of a pictograph.
See also: point of origin
KEY EPISODES
"Stargate" the Movie - Daniel Jackson describes the need for the seventh symbol in the gate address.