Really, it isn’t. I promise.
“Double knitting” means producing a piece of fabric that does not have a “wrong” side. It is a fabric where both sides are “front” sides. For instance, if you have a sweater made with more than one color yarn, you might have noticed that the inside of the sweater does not look the same as the outside – on the inside, strands of yarn run along, so you can’t actually see the pattern. It’s not very pretty. That sweater is not double knit, since no one is meant to see the inside of it. But a double knit item, even using two or more colors, does not have that. You can see the pattern on both sides, though one side will have the dark yarn as the background color, and the other side will have the light yarn as the background color.
Basically, double knitting is knitting in the round squashed flat, if you can picture that. Picture knitting a hat, or socks. You are knitting in the round, using circular or double pointed needles. If you flattened that round circle in your hands and then looked at both sides, you can see how an item could be knit so that both exposed sides are the front side. The color changes with the unused yarn being carried along is squashed on the inside, where no one sees.
That is the essence of double knitting. You knit in pairs of stitches, always a knit stitch first, then a purl stitch second (you might recognize that knit one, purl one pattern – it’s normally called 1×1 ribbing). And you knit with two colors, always the knit stitch in one color, the purl stitch in the other color.
If you did only that, and did not cross the yarn at the end of the row, you would end up with two separate pieces of fabric for your effort – one in one color, the other in the second color. But there’s really no point in that. The magic of double knitting comes in using the two colors to create a design in your knitting. Still sticking to pairs of stitches, knit one, purl one, and making sure that you use one color for the knit stitch, and the other color for the purl stitch, you can create any two color design you want – like a heart or a tree or a hobbit hole or a tardis or any abstract or geometric design. Cross the two yarn colors at the end of the row, and the fabric holds together as one piece.
You know that a knit stitch, if you turn it around and look at it, is a purl stitch on the other side, and a purl stitch, if you turn it around, is a knit stitch on the other side, don’t you? If not, surprise! Purl stitches are just backward knit stitches. Every time you make a little V of a knit stitch, the loop you put to the back is the bar of a purl stitch. That’s knitting magic and it is what makes double knitting work the way it does.
Interestingly, getting back to double knitting, though the stitches are all in a straight line on your needles, knit, purl, knit, purl, little pairs of soldiers all down the row, as soon as you start knitting the next row, the pairs of stitches coming off the needles line up one behind the other (almost – there is a slight offset). On either side of the fabric you are making, knit stitches are the side you see, and the purl sides are back to back on the inside of the fabric. This means double knitting is thicker (and warmer) than other kinds of knitting, and the back-to-back stitches means that double knitting will not curl like normal garter stitched items, where only knit stitches are on the face of the knitting. And voila! You now know how to do it your own self.
I hope that was clear. If I did my job right, you should be able to picture double knitting in your head right now and see that it is not hard. Yes, it’s still magic, I agree. But it’s not hard.
It’s easier than knitting lace. I always get lost trying to knit lace.