The White, Cãlugãreanu, Fuller Theorem

1. A doubly-encircled band is topologically equivalent to a band with T=2.
2. At any point on this band, the total twist (Linking number) can be decomposed into:
   Lk = T + W (T=Twist, W= Writhe; W measures the extent to which coiling of the central curve has relieved local twisting of the cord)
The Writhe is the quantity that connects the manifold of a single band in 2D space (T=2, W=0) with the manifold of a doubly-encircled band (T=0, W=2, also in 2D space) by projection into 3D space (T, W, > 0)
3. Positive values of W are termed overtwisting by supercoiling
4. Negative values of W are termed undertwisting by supercoiling

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.