The author has measured the strong-coupling constant alpha S from an analysis of particle clusters in the reaction e+e- to hadrons at centre-of-mass energies near the Z resonance. After the Z decays into a quark-antiquark pair, the quarks radiate gluons, which themselves decay into gluons or quark-antiquark pairs. This phase can be described by perturbative QCD. At a certain point the partons are transformed into colour neutral hadrons (hadronization). This phase cannot be described by perturbative QCD, but rather by phenomenological models.