Time evolution of a few properties of the two BHs (each of them is depicted by either blue solid or orange dotted lines) in the idealised galaxy merger simulation. From top to bottom: separation distance between the two BHs; BH dimensionless spin parameter; BH mass; x,y, z component of the unit vector of the BH angular momentum. Left panel: entire simulated timespan. Right panel: timespan of 2 Myr, centred on the time of the merger (grey, dotted vertical line). The vertical green dash-dotted and purple dashed lines mark the instants in time illustrated in the top and bottom panel of Fig. 7, respectively.