The 3rd octave chatter in cold rolling mills is a major issue for theindustrial production of thin sheets with very tight tolerances, likepackaging steels for instance. In this context, an experimental andtheoretical approach, completed in partnership by Arcelor and VAI,afforded a precise analysis and innovative results, in the followingfields : global on-line tandem mill instrumentation and efficient signaltreatments implementation ; clear differentiation between brummer (non divergent mode) and chatter (divergent mode) ; modelling ofinteractions between chatter and technology (analytical and F.E.approaches) ; advanced chatter monitoring and industrial implementation.