  Gauge systems in the confining phase induce constraints at the boundaries of
the effective string, which rule out the ordinary bosonic string even with
short distance modifications. Allowing topological excitations, corresponding
to winding around the colour flux tube, produces at the quantum level a
universal free fermion string with a boundary phase nu=1/4. This coincides with
a model proposed some time ago in order to fit Monte Carlo data of 3D and 4D
Lattice gauge systems better. A universal value of the thickness of the colour
flux tube is predicted.
