The determination of ${\alpha }_s$ 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 $\alpha >0.2$ and at the same time has the physical scales small compared to the cutoff $1\mathrm{/}a$ in all stages of the computation. The result \Lambda_\overline{MS}^{(3)}=332(14)~MeV is translated to \alpha_\overline{MS}(m_Z)=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 $0.0002$.