Distribution of cluster pairs in the space of age difference vs. ratio of R_roche to the tidal radius for each cluster. The sub-clusters of a pair are shown in the same colour and shapes. When two sub-clusters of a pair differ in age by less than 50 Myr (horizontal dotted black line) and the ratio of R_roche to tidal radius of each sub-cluster is lower than 1 (vertical dotted black line), it is considered a PBOC.