The determination of αs by the ALPHA collaboration

We review the ALPHA collaboration strategy for obtaining the QCD coupling at high scale. In the three-flavor effective theory it avoids the use of perturbation theory at α∼>0.2 and at the same time has the physical scales small compared to the cutoff 1/a in all stages of the computation. The result ΛMS‾(3)=332(14)MeV is translated to αMS‾(mZ)=0.1179(10)(2) by use of (high order) perturbative relations between the effective theory couplings at the charm and beauty quark “thresholds”. The error of this perturbative step is discussed and estimated as 0.0002.