  An infrared and collinear (IRC) safe definition of the partonic flavour of a
jet is vital for precision predictions of quantum chromodynamics at colliders.
Jet flavour definitions have been presented in the literature, but they are
typically defined through modification of the jet algorithm to be sensitive to
partonic flavour at every stage of the clustering. While this does ensure that
the sum of flavours in a jet is IRC safe, a flavour-sensitive clustering
procedure is difficult to apply to realistic data. We introduce a distinct and
novel approach to jet flavour that can be applied to any collection of partons
defined by any algorithm. Our definition of jet flavour is the sum of flavours
of all partons that remain after Soft Drop grooming, reclustered with the Jade
algorithm. We prove that this prescription is IRC safe through
next-to-next-to-leading order (NNLO), and so can interface with the most
precise fixed-order calculations for jets available at present. We validate the
IRC safety of this definition with numeric fixed-order codes and further show
that jet flavour with Soft Drop reclustered with a generalised kT algorithm
fails to be IRC safe at NNLO.
