Approximately 10 per cent of star clusters are found in pairs, known as\nbinary clusters. We propose a mechanism for binary cluster formation; we use\nN-body simulations to show that velocity substructure in a single (even fairly\nsmooth) region can cause binary clusters to form. This process is highly\nstochastic and it is not obvious from a region's initial conditions whether a\nbinary will form and, if it does, which stars will end up in which cluster. We\nfind the probability that a region will divide is mainl…