Jets are the most abundant objects at the LHC. While their formation has been well-established for over two decades, the physics community has recently taken a strong interest in understanding the underlying substructure of jets. In this talk I will focus on a specific observable, the subjet multiplicity, whose precise description presents a long-standing theoretical issue that limits the ability to use quark- and gluon-discrimination for jets and which can offer fundamental insights into the perturbative structure of jets. In the talk I will present a new, collider-generic, definition for subjets multiplicity called the Lund multiplicity and present recent developments for predicting the average Lund multiplicity in perturbative QCD and outline its impact on phenomenology at the LHC and LEP.
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