Sock Dye Blanks

Here is the basic idea, 2 strands of the same yarn are knitted together into a strip of fabric. This fabric is then dyed in any number of different ways, stripes of color, tie-dyed, or perhaps printed with dye. Then, a knitter will knit one sock from each of the two strands that run throughout the fabric. This ensures a fairly even, symmetrical pattern from one sock to the other.

This picture is from Knit Picks, where they sell the blank pre-knitted fabric for customers to dye themselves: