  When a theory shall be described at all scales, it is necessary to start from
its elementary degrees of freedom. Herein, one possible chain of steps for this
purpose will be briefly outlined for the example of a gauge theory, like QCD.
Starting with the elementary constituents, gluons and quarks, step by step the
final observables, physical states, will be built up. This process is based on
the elementary correlation functions, and uses a combination of both numerical
lattice calculations and functional continuum methods. While the process uses a
fixed gauge at each intermediate step, the final observables are
gauge-invariant.
