  Recent proposals to re-define some of the base units of the SI make use of
definitions that refer to fixed numerical values of certain constants. We
review these proposals in the context of the latest results of the
least-squares adjustment of the fundamental constants and against the
background of the difficulty experienced with communicating the changes. We
show that the benefit of a definition of the kilogram made with respect to the
atomic mass constant (mu) may now be significantly stronger than when the
choice was first considered 10 years ago.
