  A new kind of Lagrangian diagnostic family is proposed and a specific form of
it is suggested for characterizing mixing: the maximal extent of a trajectory
(MET). It enables the detection of coherent structures and their dynamics in
two- (and potentially three-) dimensional unsteady flows in both bounded and
open domains. Its computation is much easier than all other Lagrangian
diagnostics known to us and provides new insights regarding the mixing
properties on both short and long time scales and on both spatial plots and
distribution diagrams. We demonstrate its applicability to two dimensional
flows using two toy models and a data set of surface currents from the
Mediterranean Sea.
