  A quantitative description of the qualitative feature of multi-hadron final
states known as the "number of jets" is given by a sequence of infrared finite
shape observables (jet discriminators) that: take continuous values between 0
and 1; are stable-unlike clustering algorithms-against small variations of the
input (data errors, Sudakov effects etc.); have a form of multiparticle
correlators that is natural in the context of quantum field theory and hence
are better suited for a systematic study of theoretical uncertainties
(logarithmic and power corrections).
