Central density ρ_c as a function of core radius r_c for clusters with ρ_c > 2000 L_⊙/pc³ (Harris 1996, update of 2010), with lines of constant Γ (blue) and constant γ (red). Letter labels refer to clusters in Table 1, coloured red if N_s > N_b, blue if N_b > N_s, and yellow if N_b ≃ N_s. Clusters with products of secondary interactions include i) and g), and – indicated in black, with numerical labels – 1: NGC 1851, 2: NGC 6342, 3: NGC 6397 and 4: NGC 6544.