This letter proposes a criterion to quantitatively correlate chemical composition with crystallization behaviors of Al-based glass-former alloys. By extending to multicomponent systems the topological instability model of Egami and Waseda [J. Non-Cryst. Solids 64, 113 (1984)], we introduce a “lambda $\left(\lambda \right)$ criterion” according to which amorphous alloys with $\lambda >0.1$ exhibit glassy behavior, whereas alloys with $\lambda <0.1$ are nanocrystalline. Nanoglassy alloys occur when $\lambda \approx 0.1$ . A large number of experimental findings in the literature supports the $\lambda$ criterion, rendering it a useful tool for the design and selection of glass-former systems and compositions.