Threshold Effects on the QCD Coupling $\alpha_{\overline{MS}}$
  The matching condition which determines the effect of a heavy quark threshold on the running of the QCD coupling $\alpha_{\overline{MS}}$ is reviewed. The matching scale is arbitrary to some extent. However, this affects the value of $\alpha_{\overline{MS}}$ away from the threshold region only marginally. 
W. Bernreuther