Jazz must swing - jazz musicians agree on that. However, even 100 years after the beginnings of jazz, it is still unclear what exactly constitutes the swing feel. With a sophisticated experiment and data analyses on more than 450 well-known jazz solos, physicists from the Max Planck Institute for Dynamics and Self-Organization (MPI-DS) together with psychologists from the University of Göttingen have unraveled a secret of swing. They were able to demonstrate that certain systematic deviations in timing are a key component of swing. These microtiming deviations are so small that they are not perceived as such even by professional jazz musicians, who nonetheless are using them unconsciously.