Velocities derived from the peak separation of [N ii] (diamonds) and [O iii] lines (triangles) against V_exp, the expansion velocities given by Frew (2008), Tables 3.8 (large symbols) and 9.4 (small symbols). The dashed line is the 1:1 relation. Circled symbols indicate objects for which peak separation velocities from both ions are available. Open symbols mark objects from the local sample with distance ≤1 kpc, filled ones PNe with distances >1 kpc.