The international definition of 'traceability' is the... property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.

As said above, mass is unique amongst the base quantities of the SI because its unit definition, the kilogram, is based on a physical artefact; a cylinder of platinum iridium alloy, held at the Bureau International des Poids et Mesures BIPM near Paris, is defined as being exactly one kilogram in mass. All mass measurements undertaken in the World should be traceable to this single artefact - the international prototype of the kilogram (known as K - see history of the kilogram) - and this is achieved by regularly comparing its mass with the official 'copies' of the Kilogram held in national measurement institutes, such as NPL.

The official copies have masses that differ by up to nearly 1 mg; they are certainly not identical, either to each other or to K, and unfortunately (and more importantly) all change their mass with time, albeit not by very much. Great effort is put into understanding the differences and changes, and trying to compensate for them to ensure continuity and consistency of mass traceability.

The United Kingdom's national standard of mass - known as Kilogram 18 - is held at NPL and is the basis of the mass scale, or traceability hierarchy, in the UK. NPL participates in a wide range of international comparisons to ensure that mass measurements made in the UK are equivalent to those made elsewhere.

Is traceability important? Yes it is; without traceability it would not be possible to ensure that one person's idea of 'a kilogram' was comparable with another person's version. Particularly in applications demanding substantial accuracy - and modern technology provides a huge range of them - it is surprisingly easy to inadvertently end up with the wrong values through inadequate attention to the detail of traceability.