  We show that one can express the knot equation of Skyrme theory completely in
terms of the vacuum potential of SU(2) QCD, in such a way that the equation is
viewed as a generalized Lorentz gauge condition which selects one vacuum for
each class of topologically equivalent vacua. From this we show that there are
three ways to describe the QCD vacuum (and thus the knot), by a non-linear
sigma field, a complex vector field, or by an Abelian gauge potential. This
tells that the QCD vacuum can be classified by an Abelian gauge potential with
an Abelian Chern-Simon index.
