I was watching MTV World Stage, and Slash had a DOUBLE guitar. What is this purpose? Different key? Sound?

As Danny says, most double necks are a 6 and 12 string guitar, but I've also seen 12 string and a bass, two six string, and 6 string and bass.

Now...in the "classic 6/12 layout, the 12 string guitar has two strings for each note..the EADG have a second set alongside, much like a mandolin set up, but one set are very light gauge and tuned an octave higher and the B and E strings are tuned the same. (On my first 12 strings acoustic I found and restored, I didn't know about the B and E being the same and tuned up an octave on those, too, lucky I didn't put an eye out!!)

So the effect is like two different guitars with different octaves playing at the same time...The chorus effect can be used to kind of simulate that sound on a 5 string electric.

The 6 string is just an SG,.

Yes it is heavy!! about 22 lbs and tends to drop down on the headstock side, so I had to buy a padded bass strap about 3-4 inches wide, which distributes the weight and makes it tolerable to play. Not recommended for long stints..a song here, a song there...when the effect of a 12 strings is needed in parts, (as Danny said, Page used it to play Stairway to Heaven live)

I bought mine because I grew up as a teenager worshipping John McLaughlin in the early 70's...a few years before Page!

It is really not a useful as I'd hoped, and even with the humbuckers, I tried using the 6 string neck to record a Rush cover, but sitting in my home studio for half a day, trying to work around the albatross hanging on my neck...I recorded about half the song using it, then used my Strat to just edit a couple of parts and found I really couldn't tell that much difference is distortion, but the Strat was "cleaner" , so I re-recorded the whole song with just the strat.,

Plus, standing up the 6 string part hangs a bit low and is out of my preferred playing angle., and the 12 string is kind of high up my chest, higher than I like....so..to sum up..it's a really neat novelty and does have it's uses, it would work better with a tube amp, where humbucker gain actually does have a bit more punch to it..compared to a strat, but using any effects negates that advantage...

So ..I actually don't play it all that much...but I'd wanted one since 1974 and finally had a chance to buy one in 2008.

Slash Double Guitar

Well, You Could Try Asking A Music Store. Other Than That Go To A Place Like Guitar Center And Ask About Foot Pedals And Effects. Theres A LotOf Stuff Out There. But Ya Try Asking A Music Store

Likely a double-neck guitar, most often with a 6-string neck and a 12-string neck. Instant choice of sounds, and a good "wow" factor, but a little hard on the shoulder (they're pretty heavy). Russell E. has one, and hopefully will show and tell. PS, Jimmy Page, L Zep, was using one way before Slash, and the first on a rock stage, as I remember.