Double knitting - pictures
My first attempt. Part of a sampler from a class with Britt-Marie Christoffersson in 2002. A small pocket where the number of stitches has been doubled to form two layers. A section from the front layer has been cast off to form an opening, while the stitches on the sides have been knit together.
A few years later I made a Moebius shawl in Raggi by Järbo. I wanted make a hat and mittens to go with it, but didn't have any dpns in the right size. Then I made these hats and mittens on straight bamboo needles 5mm in double knitting in the same technique as the sample above, but with a regular cast on that was folded double to make it open at the bottom.
This sample shows what double knitting with one strand looks like in progress. The stitches on one side have been divided onte two needles to demonstrate the opening inside.
This is double knitting in two colours. Cast on and cast off have been made in double yarn with both colours. The edges were knit in double yarn and lifted to form a chain. The colours have changed places in some places to make a pattern.
And this is the other side, with the opposite colourway and the pattern mirrored.
This is the same technique as above, but with different edges. Tubular cast on and cast off gives the impression that the piece has no beginning or end. At the edges the colours have just been wrapped around eachother at each turn.
And here the back looks the same since cast on and cast off look the same on both sides and so does the pattern.
Double knitting - techniques
There are two main directions in double knitting. Either you knit two layers at the same time on straight needles with one thread, e.g. for a pocket, pouch, bag, hat och mitten. The other use is to create a piece in two colours, with the same pattern on both sides but with opposite colours. This can be used for pot holders, cushions, scarfs etc. In both cases it is a good idea to use slightly thinner needles than recommended for the yarn, since the knitting becomes looser than usual. This is especially important when knitting in two colours, because then the fabric should be thick enough so that the colour on the back doesn't shine through.
With one colour
Cast on: Cast on twice as many stitches as needed for the width, since every other stitch will be at the front and every other on the back. The number of stitches should be even. If the bottom should be closed, e.g. for a bag, cast on as usual on one needle. You can also cast on half
the number of stitches required and then knit one, purl one into each
stitch and double the number in that way.
If the bottom should be open, e.g. for a hat, cast on on two needles, one after the other, and then place these parallell to eachother and knit the last, lift the first, knit the second form the end, lift the second etc. until you reach the middle of the piece and the stitches from the two needles have come together like a zipper.
Knitting: To make a circular tube, knit all stitches. On the first row knit the first, third, fifth stitch etc. The other stitches are lifted as if to purl. After turning the piece, repeat the same method, but now knit the stitches that were lifted on the first row and vice versa. For stockinette it is easiest to hold the yarn at the back the whole time and knit so that the outside of the tube looks purled and turn the tube inside out when finished knitting. If you like the purled surface, don't turn. You can also make patterns in knit and purl, it is just a little bit trickier. It is very important that the yarn runs the right way and the two layers don't hook into eachother. If that happens the layers can't be separated and ripping back is the only solution.
Cast off: If you don't want an opening at the top and don't want to turn it inside out, just cast off. Knit one stitch from the front with one from the back throughout the cast off. If you want to reverse the tube (and there is no opening in the bottom) or if you want an opening at the top the stitches have to be moved onto two needles, one for the front and one for the back, before they are cast off.
With two colours
Cast on: Cast on twice as many stitches as needed for the width, since every other stitch will be at the front and every other on the back, using only one thread. You can also cast on with double yarn, one strand of each colour, and knit each tread as one stitch. Then there is no need to double the number of stitches since each stitch cast on is knit as two stitches. When knitting with two colours the layers should stick together and there is no need to separate them or open the ends.
A third method is a tubular cast on. Make a temporary cast on and knit at least four rows in one colour. Then the temporary cast on is removed and these stitches are placed on a new needle. The needles are then placed parallell and you take every other stitch from the two needles until they have come together like a zipper.
Edge: Both strands have to go all the way out to the sides all the time. Therefore the edges are knit in double yarn. You can either knit one or more edge stitches on each row or lift the edge stitch on every other row to make a chain along the edges. You can also knit as usual al the way to the end of the rows, but then the strands must be twisted round each other every time the work is turned.
Background: The actual double knitting is made as a combination of knit one, purl one rib and two coloured knitting where both strands are held together. First hold both strands behind and knit the stitch that will be on the front side. Then both strands are moved to the front and the next stitch is purled using the second colour. Continental knitters should hold the strands over the index finger with the knit colour to the right and the purl colour to the left. Turn the work and knit the other side in the same way, but change the colours. You can also knit in the round. Then the same side is always at the front, but otherwise it is the same method but you can leave out the edge stitches.
Pattern: By knitting stitches in the "wrong" colour you can create patterns in two colours. Let the strands change places and knit and purl the desired number of stitches, then switch back and continue with the background. Don't forget to do both the knitted front stitch and the purled backside stitch in the same manner so that the pattern becomes the same on both sides. Since no threads are left hanging on the back the patterns can be placed far apart, but the pattern is mirrored on the other side so it should be symmetrical.
Cast off: Cast off using single or double yarn to match your cast on. To match a tubular cast on, divide the back and front stitches and knit one row less than at the cast on over half the stitches using one colour, then graft the front and back stitches together.
Patterns
The only patterns I know of in English are a scarf, a hat and a baby blanket at http://www.knitty.com
More ideas in Swedish and Norwegian can be found at the Swedish version of this text.
Good Luck!