• White (1969): A singly encircled band with two half-twists is topologically equivalent to a doubly-encircled untwisted band.
• Cãlugãreanu/Fuller: The Linking number describes both forms, and comprises;
  Lk = T_w + W_r
  W_r or Writhe measures the extent to which coiling of the central curve has relieved local twisting T of the ribbon (T_w is ∫T)
• • Positive values of W_r are termed overtwisting by supercoiling
  • Negative values of W_r are termed undertwisting by supercoiling

The White, Cãlugãreanu, Fuller Theorem; J. H. White, Am. J. Math., 1969, 91, 693-728; G. Cãlugãreanu, Czech Mathematics J., 1961, 11, 588-625; F. Fuller, Proc. Natl. Acad. Sci., 1971, 68, 815-819.