Stochastic-Alpha-Beta-Rho model for the forward F and its vol α: dF_t = α_t,F_t^β,dW_t^F, dα_t = nu,α_t,dW_t^α, dlangle W^F, W^αrangle = ρ,dt. Hagan's expansion gives an approximate implied-vol formula in (α, β, ρ, nu) that is the swaption-market standard for quoting smile/skew. β controls backbone (lognormal β=1, normal β=0), ρ controls skew, nu controls smile curvature.