Arithmetic Brownian motion for the price level: dS_t = μ,dt + σ_N,dW_t, so S_T is normally distributed (additive, not log-additive). Yields a closed-form call price C = (S - K)Phi(d) + σ_N√(T),φ(d) with d = (S-K)/(σ_N√(T)). Permits negative prices, which is exactly why it became the post-2020 standard for SOFR caps/floors and oil options after WTI futures printed -$37 in April 2020. Bachelier vol σ_N and Black vol σ_B are related by σ_N ≈ σ_B · F ATM (where F is the forward).