Assembly bias in the Lagrangian b_K²(b₁) relation for halos split into four quartiles by spin (at fixed mass). All lines use the second-order spatial estimator and k_d = 0.25 h Mpc⁻¹ as damping scale. Dashed lines show the variation of b₁ and b_K² at fixed mass. The b_K² − b₁ relation shows a strong degree of assembly bias – with larger spin selections yielding larger b_K² and larger b₁.