  This talk discusses two topological features in non-abelian gauge theories,
related by the notion of abelian projection and the Hopf invariant. Minimising
the energy of the non-linear sigma model with a Skyrme-like term (the
Faddeev-Niemi model), can be identified with a non-linear maximal abelian gauge
fixing of the SU(2) gauge vacua with a winding number equal to the Hopf
invariant. In the context of abelian projection the Hopf invariant can also be
associated to a monopole world line, through the Taubes winding, measuring its
contribution to topological charge. Calorons with non-trivial holonomy provide
an explicit realisation. We discuss the identification of its constituent
monopoles through degenerate eigenvalues of the Polyakov loop (the
singularities or defects of the abelian projection). It allows us to study the
correlation between the defect locations and the explicit constituent monopole
structure, through a specific SU(3) example.
