Stargate Codes

[Cash]

Im just curious about what each code means on base,
example; code 9, and how many codes and what they mean.

[Cash]

and when do they ever change the DEFCON?

[Xaeden]

Im just curious about what each code means on base,
example; code 9, and how many codes and what they mean.

Code 3 - Carter declared it in "Rite of Passage" (Season 5, Episode 6) after Cassandra revealed there was a Goa'uld on the base. We never saw it activated in any other episode, but it probably broadly means that a hostile has infiltrated the base. There was no corresponding lockdown as far as we know.

Code 5 lockdown - Heard over the loudspeakers in "The Broca Divide (season 1, episode 5) after Hammond told the president that he ordered the base to be locked down and anyone attempting to escape to be shot on sight in order to contain a contagious disease.

Code 9 - O'Neill called it in "Spirits" (Season 2, Episode 13) because "SG-11 is boppin' around, waving their arms, makin' our people disappear" and Hammond did so in "Fair Game" (Season 3, Episode 3) immediately after seeing O'Neill disappear in front of him (Thor beamed him to his ship). It doesn't specifically signal that base personnel are disappearing, though, as in "Show and Tell" (Season 2, Episode 20) Carter activated it by hitting a button after finding out that a Reetou may have snuck onto the base. From this we know that it's a type of lockdown as the computer said "Lockdown in progress," after Carter hit the button. Whether it also signals that a hostile force is on or is suspected of being on base is unclear.

Code 17 - Appears to indicate suspicion that a Goa'uld has taken over someone on base as Hammond said that Frasier declared it in "Lifeboat" (Season 7, Episode 6) after Jackson came back from an alien ship with multiple personalities. Carter responded to Hammond by saying, "She thought he was a Goa'uld?"

Also...

Threatcon Bravo - Frasier put the base on this in "Learning Curve (season 3, episode 5) after explaining that she discovered millions of nanites in Merrin's brain. She then ordered another doctor to get blood samples from everyone because she wanted to "know how far those nanites have spread."

Class One security lockdown - Hammond ordered it in "Sight Unseen" (Season 6, Episode 13) after Jonas said he saw an alien creature disappear into the wall. It's unclear how this differs from other lockdowns, but it seemed serious as Hammond went out of his way to assure everyone that it was not a drill.

and when do they ever change the DEFCON?

A few times...

"Nemesis" (Season 3, Episode 22)
Davis: "General, we're going to DEFCON 3. The Russians have gone on alert as a response. The moment the Asgard ship attempts to land, the President had decided to let the rest of the world know what we know."

Davis was referring to the Thor's replicator infested ship that was orbiting the planet.

"Entity" (Season 4, Episode 20)
HAMMOND: "I'm maintaining DEFCON 2 and keeping the mountain locked down until you can tell me with confidence there will be no further repercussions."

This was odd considering it was early in the episode and all that happened was an alien entity accessed the SGC's computer network but had been stopped before it could complete an entire scan.

"Lost City" (Season 7, Episode 22)
After detecting three Goa'uld ships exit hyperspace President Hayes told Hammond to remain at DEFCON 3. Another 30+ motherships appeared in orbit later in the episode, but Hayes only ordered they go to DEFCON 1 two hours later when General Maynard reported that the Goa'uld had been systematically taking out power grids and broadcast channels globally since they showed up in orbit.

"Full Alert" (Season 8 Episode 14)
It was counted down as the likelihood of war between Russia and the United States increased (due to Goa'uld meddling). O'Neill first announced that they went to DEFCON 3 toward the middle of the episode, then DEFCON 2 a little later, and finally DEFCON 1.