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Fundamental constants link seemingly different fields of physics and seemingly different quantities and measurement units. Consequently, they are of the utmost interest in metrology and it has been planned to redefine the kilogram by fixing the numerical value of the Planck constant. The paper by H. Bettin et al. (pp. 680–687) summarises the methods to measure the ratio between the Planck constant and a mass and reviews the determination of the Avogadro constant by counting the atoms in a silicon crystal highly enriched by the 28Si isotope.