Calibration chains
Fundamental Metrological Standards

At the international level, decisions concerning the International System of Units (SI) and recommendations concerning the realization of primary standards and the implementation of the SI are taken by the General Conference on Weights and Measures (CGPM). The Bureau International des Poids et Mesures (BIPM) is responsible for coordinating and implementing the actions defined by the Comité International des Poids et Mesures (CIPM) and the CGPM, and for organizing inter-laboratory comparisons at the highest level (known as key comparisons). In addition, one of the BIPM's special features is to produce International Atomic Time (TAI) from clocks belonging to national metrology institutes, and to make TAI available to users.

The fundamental principle of traceability (figure