Normalised total disc torque Γ∕Γ₀ versus time t, where , q = M_em∕M_⋆, for four events studied in detail: a merger (top panel), a co-orbital formation, a repulsion, and an exchange (bottom panel). These events were selected from the following simulations: CaseIII_nominal, ditto, Sigma_3times, viscosity_1e-6. Only those embryos which take part in the interaction are plotted. The label order corresponds to the (initial) radial distance. Each case exhibits a very different progression of the torque. In particular, there is (i) strong negative torque acting on the outer embryo (for the merger); (ii) positive torque on the inner embryo (co-orbital); (iii) alternating torques on both (repulsion); and (iv) negative torque on the outer and positive on the inner (exchange).