Double Knitting. Not the weight of the yarn, but the method of knitting a double layer of fabric.
In essence, to knit a double layered fabric, you knit both layers at the same time, alternating stitches from the near layer and the back layer. You don't have to have both right sides as stocking stitch, you don't even have to have an even number of stitches. You can knit both layers using just one yarn, orrn or two different yarns. You can knit a tube or a doulbe layer of fabric.
I've never used double knitting to knit a tube, but I can imagine it beign very useful for things like glove fingers. You have double the stitches (as you're knitting two layers). You treat alternative stitches as belonging either to the front/near side or the back/far side, and (if you're working in one colour) would (k,sl) across the row. When you turn your work you knit the slipped stiches and slip the knit stitches, from the previous row. As long as you don't inadvertently catch part of the far layer as you knit you will end up with a tube.
I've only done two colour double knitting, where you don't do (k, sl) but (k,p) across the row. You trat all knitted stitches in the row as belonging to the near side layer, and the purl sts as belonging to the far side layer (which is ws facing on this row, therefore the stitches are purled). Presuming you are of course doing stocking stitch. You can add patterns by swapping the colours around.
I've only knitted two things (!) in double knitting. First off, Cheesehands, which are knit in the round.

and this baby bonnet.


I loved knitting both of these, but have to say that I'm terrified of begining another double knitting project!
I have a few in the queue - a couple of pairs of hats, three pairs of mittens and a few scarves.
Yesterday I thought I'd cast on a scarf, the Bubbles Scarf from Lucy Neatby. It's similar to these two above, using negative colours, but the bubbles are stuffed!
As usual, the swatching went badly. I really only do this to get back intot he swing of double knitting and reminding myself of my weak points, usually the row ends and knitting two stitches togetrehr, which can be easily rectified on the next row but is annoying.
Hence my swatch is all over the place, and unfinished.


That's meant to be a circle, but I changed needle sizes and swapped the colours around mid-circle. I stopped before the end as I wanted to make sure that the circles hadn't been joined and that I would be able to stuff them!
And the knitting. SLOW.

Guaranteed, this won't be a quick project. I anitcipate it taking many months!