A Note on Proton Stability in the Standard Model

In this short note we describe the symmetry responsible for absolute, nonperturbative proton stability in Standard Model. The SM with $N_c$ colors and ${N_g}$ generations has an exact, anomaly-free, generation-independent, global group $U(1)_{B-N_c L} \times \mathbb{Z}_{N_g}^L$, which contains a subgroup of baryon plus lepton number order $2 N_c {N_g}$. This disallows decay ${N_g}>1$. Many well-studied models beyond explicitly break symmetry, alternative deserves further attention.