Locations of member stars with different internal motion directions on the projected coordinate system, whose origin is located at the common center of the Double Cluster core. With the mean tangential velocity of core members as its zero point, stars are categorized into four samples according to their residual tangential velocity direction, as plotted with different colors. Arrows close to the label of S1 and S2 point out the average internal motion direction of substructure members. The almost orthogonal motion direction suggests the possibility of different dynamical formation histories of these two substructures.