Linking number (Lk). Each blue ribbon represents one strand of a double-stranded DNA molecule. (a) The linking number of the molecule is 1. (b) The linking number is 6. One of the strands is kept untwisted for illustrative purposes, to define the border of an imaginary surface (solid blue oval). The number of times the twisting strand (its relative length exaggerated here) penetrates this surface is the linking number.