of a decades-long move towards increasingly abstract and idealised formulation in which the realisations of the units are separated conceptually from the definitions. The great advantage of doing it this way is that as science and technologies develop, new and superior realisations may be introduced without the need to redefine the units. Units can now be realised with an accuracy that is ultimately limited only by the quantum structure of nature and our technical abilities but not by the definitions themselves. Any valid equation of physics relating the defining constants to a unit can be used to realise the unit, thus creating opportunities for innovation... with increasing accuracy as technology proceeds.' In practice, the CIPM Consultative Committees provide so-called "mises en pratique (practical techniques), which are the descriptions of what are currently believed to be best experimental realisations of the units. This system lacks the conceptual simplicity of using artefacts (referred to as prototypes) as realisations of units to define those units: with prototypes, the definition and the realisation are one and the same. However, using artefacts has two major disadvantages that, as soon as it is technologically and scientifically feasible, result in abandoning them as means for defining units. One major disadvantage is that artefacts can be lost, damaged, or changed. The other is that they largely cannot benefit from advancements in science and technology. The last artefact used by the SI was the International Prototype Kilogram (IPK), a particular cylinder of platinum-iridium; from 1889 to 2019, the kilogram was by definition equal to the mass of the IPK. Concerns regarding its stability on the one hand, and progress in precise measurements of the Planck constant and the Avogadro constant on the other, led to a revision of the definition of the base units, put into effect on 20 May 2019. This was the biggest change in the SI since it was first formally defined and established in 1960, and it resulted in the definitions described above. In the past, there were also various other approaches to the definitions of some of the SI units. One made use of a specific physical state of a specific substance (the triple point of water, which was used in the definition of the kelvin); others referred to idealised experimental prescriptions (as in the case of the former SI definition of the ampere and the former SI definition (originally enacted in 1979) of