To measure anything accurately - hence to obtain a measurement result or assign a value that is as true, correct or exact as possible - and for this measurement to be repeated anywhere in the world and yield an accurate result, a system of units such as the International System of Units, the SI, is needed. In order to establish such a system, a system of quantities including a set of equations defining the relations between those quantities, must be established. Corresponding units must then be defined as a measure of the quantities. The units must be unique and provide a sound theoretical basis upon which the most accurate and reproducible measurements can be made. Procedures to establish the value and associated uncertainty of the same kind as the unit must be established, called the "realisation of the definition of a unit". Finally, these realisations and the measurement standards (including "weights") established to disseminate the realisations in countries across the world, called national measurement standards (NMS), must be compared to determine their degree of equivalence (or comparability