
The figure below represents some of the links between the base units of the SI (shown in red circles) and the fundamental physical and atomic constants (shown in green boxes). It is intended to show that the base units of the SI are linked to measurable quantities through the unchanging and universal constants of physics.
In the figure,

the surrounding boxes, lines and uncertainties represent measurable quantities. The numbers marked next to the base units are estimates of the standard uncertainties of their best practical realizations; those next to the fundamental constants represent the uncertainty of our knowledge of these constants (from the 2002 CODATA adjustment). 

the grey boxes reflect the unknown longterm stability of the kilogram artefact and its consequent effects on the practical realization of the definitions of the ampere, mole and candela. 
The definition of the ampere, for example, involves the kilogram, but an alternative link is the Josephsoneffect constant (K_{J90}) and von Klitzing's quantumHall resistance (R_{J90}), both of which were given fixed, conventional values in 1990.

See also
Quinn T.J., "Base units of the Système international d'unités, their accuracy, dissemination
and international traceability", Metrologia, 1995, 31, 515527;
Kind D., Quinn T.J., "Metrology: Quo Vadis?", Physics Today, August 1998.


