A knot property, also called the twist number, defined as the sum of crossings of a link ,

(1)

where defined to be if the overpass slants from top left to bottom right or
bottom left to top right and
is the set of crossings of an oriented link.

The writhe of a minimal knot diagram is not a knot invariant, as exemplified by the Perko pair, which
have differing writhes (Hoste et al. 1998). This is because while the writhe
is invariant under Reidemeister moves II and
III, it may increase or decrease by one for a Reidemeister move of type I (Adams
1994, p. 153).

Thistlethwaite (1988) proved that if the writhe of a reduced alternating projection of a knot is not 0, then the knot is not amphichiral
(Adams 1994).

A formula for the writhe is given by

(2)

where is parameterized by for along the length of the knot by parameter , and the frame associated with is

(3)

where is a small parameter, is a unit vector field
normal to the curve at ,
and the vector field
is given by

Adams, C. C. The Knot Book: An Elementary Introduction to the Mathematical Theory of Knots.
New York: W. H. Freeman, pp. 152-153 and 185, 1994.Hoste,
J.; Thistlethwaite, M.; and Weeks, J. "The First Knots." Math. Intell.20, 33-48,
Fall 1998.Kauffman, L. Knots
and Physics. Teaneck, NJ: World Scientific, p. 19, 1991.Kaul,
R. K. "Topological Quantum Field Theories--A Meeting Ground for Physicists
and Mathematicians." 15 Jul 1999. http://arxiv.org/abs/hep-th/9907119.Thistlethwaite,
M. B. "Kauffman's Polynomial and Alternating Links.' Topology27,
311-318, 1988.