Distribution of mid-plane asymmetry, A_Σ(x, z), in the edge-on projection (x − z-plane), computed separately for the thin (left columns) and thick (middle columns) discs, as well as thin+thick (right columns) disc particles using Eq. (4) at six different times (before and after the buckling event) for the model rthickE0.5. The bar is placed along the x-axis (side-on configuration) for each time-step. Black lines denote contours of constant density. A mid-plane asymmetry persists, even long after the model has gone through the buckling phase.