We show that the classical nucleation theory that describes the critical nucleus as a coherent pill-box fits the experimentally measured nucleation rate of Mg₁₇Al₁₂-precipitates in undeformed Mg-7 wt.% Al alloy much more accurately than the classical theory that describes the critical nucleus as an incoherent sphere. The nucleation of Mg₁₇Al ₁₂-precipitates takes place in the transient nucleation regime. Coarsening of the Mg₁₇Al₁₂-precipitates occurs just before the steady state nucleation rate is reached.