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The redefinition of the international system of units fixes the values of the unperturbed ground state hyperfine transition frequency of the $^133}$Cs atom, speed of light in vacuum, and Planck constant. Therefore, differently from the past, the kilogram is now explicitly defined as the unit of the inertial mass. This short communication outlines the conceptual consequences of the new definition and of its practical implementations.
Schlamminger, S.
and Mana, G.
(2022),
The kilogram: inertial or gravitational mass?, Metrologia, [online], https://doi.org/10.1088/1681-7575/ac7ca7, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=934012
(Accessed October 8, 2025)