Comparison of measurement standards (comparison): Establishing a relationship between measurement results when reproducing and transmitting a unit of measure or a measurement scale with these standards.There are:

• Key comparison: One of the set of comparisons selected by a Consultative Committee to test the principal techniques and methods of measurement in the field.
• CIPM key comparison is a key comparison executed in the framework of a
  Consultative Committee or by the BIPM. A CIPM key comparison leads to a key comparison reference value.
• RMO key comparison is a key comparison executed in the framework of a regional
  metrology organization. COOMET key comparison: A key comparison of national measurement standards organized by the NMI’s of COOMET.
• Supplementary comparison  is a comparison, usually carried out by an RMO to meet specific needs not covered by key comparisons (e.g. regional needs), for instance measurements of specific artefacts, or measurements of parameters not within the “normal” scope of the Consultative Committees. Supplementary comparisons of COOMET: Comparisons of national measurement standards, organized by the COOMET NMI’s that are executed alongside with the key comparisons.
• COOMET pilot comparison: A comparison of the NMI national measurement standards, organized by NMI COOMET and executed before a COOMET key comparison with the purpose of preliminary estimation of possible deviations of measurement results in the COOMET key comparison.

All key and supplementary comparisons are registered and their results are published in the KCDB Appendix B . Key and supplementary comparisons are  normally considered as a sufficient support for calibration and measurement capabilities (CMCs).