Double knitting is a special knitting technique. The result is an extra heavy, reinforced material with double thickness, which is still soft to the touch. The appearance is usually ribbed. Double knitting is often uses for thick, woolly jumpers. It can also be used to reinforce the heel or tip of socks in the places where they wear off most.
Double knitting involves knitting both sides of the fabric at once, alternating sides with each stitch. You work a stitch from one side, and then one from the other side. This creates a double-thick, reversible, two-layered fabric.
In the British system of yarn weights, double knitting yarn, often known as "DK", is twice the weight of 4ply yarn. It is approximately equivalent to the US light worsted weight (category 3), but be sure to work a swatch to test your gauge before starting your project!
Double knitting is a technique it is extra heavy and double thick, it is usually use for winter outfits. You knit both sides at once, you also can reinforce the heal on your socks so they will not wear off to fast, also the material is still soft to the touch.
"Double-knitting" simultaneously-knits (2)inseparable/interlocked or separate/unconnected fabrics
from (2)yarns by flat-knitting with (1)pair-of-double-pointed-knitting-needles:
Knit (1)row with (1)yarn & slide-stitches to needle's-other-end,
Knit next-row with next-yarn;
Turn work over;
Knit next-pair-of-rows;
REPEAT