I don't know that it's ever explained. No doubt it would take tremendous energy to engage the chevron... It may have to do with time travel (or other dimensions).

My personal view is that chevron 9 is to access "special functions" on the other eight chevrons, kind of like pressing "ctrl", "alt" or "shift" on a keyboard.

Correct me if I am wrong but in the original film they only used 7 chevrons: 2x3 for X, Y, Z co-ordinates and the 7th for the point of origin. It seems to me that there has been a couple of upgrades added in the meantime for extra functionality. If this keeps up they will need a complete hardware upgrade to handle all the new software, maybe it will have to be delivered in 2 rings. :D

The 'official' answer I have found at http://www.mgmuk.com/stargate-sg1/home/faq/

is as follows:
How many chevrons are on the Stargate?
The Stargate contains nine chevrons. Although the gate has nine chevrons, typically only seven of these are used when dialing from a gate. In the episode The Fifth Race, an eighth chevron was activated (thanks to the Asgard) and Carter noted, "The extra chevron must add a new distance calculation to the existing points, like a different area code." So far, the eighth chevron has been used only once (on the Earth Stargate), and the purpose or function of the ninth chevron is still unknown.

So there you go. Any speculations?

Activates a direct line from anywhere in the universe for your closest Domino's Pizza. No tipping required when you use your own gate for delivery instead of their driver.

Hey 7th_Dizbuster, carefull with those fotware upgrades, we all know what happened when Felger worked on the gate, brought the whole damn system down, quite an achievement if you ask me

I'll have to accept that the 9th chevron is just a non issue until it is used to get out of a plothole

I would venture to guess its a multi-link.
Say many gates being able to lock into the same gate. Other words say all the other gates would be able to dial into sg1 at the same time vs only 1 to 1 connection. This would allow many worlds to exit to a common point.

A guess,

maybe to differenciate between multiple gates at the same address.

Daystrom

I thought that gate connections where 1:1 as opposed to a 1:N or M:N relation

Well as much as we can speculate, I think the only way we're going to find out is if the writers get theirr fingers out and come up with some better plotlines. It's beginning to look more like a sitcom with intergalactic travel involved. This isn't to say that i'm not an SG-1 fan, I am but I can't stand it when a show reaches a certain point and the writers decide that they can let plot lines slide, GAHHHH I'M REALLY ANNOYED NOW!!!!! :evil: :evil: :evil: :evil: