Monotonic strike dependence of implied volatility — in equity indices, σ_imp(K, T) slopes downward (OTM puts richer than OTM calls), a permanent regime since the 1987 crash. Economically: crash-fear / leverage effect, with empirical SPX skew steeper at short tenors. Captured naturally by stochastic-vol with negative spot-vol correlation, by jump-diffusion (downward jumps), or by Dupire local vol calibrated directly to the surface. Skew steepness is itself traded via risk-reversal σ_25Δ p - σ_25Δ c.