 The new literature on coordination shifts the emphasis of the discussion from the definitions of quantity-terms to the realizations of those definitions. In metrological jargon, a “realization” is a physical instrument or procedure that approximately satisfies a given definition (cf. JCGM 2012: 5.1). Examples of metrological realizations are the official prototypes of the kilogram and the cesium fountain clocks used to standardize the second. Recent studies suggest that the methods used to design, maintain and compare realizations have a direct bearing on the practical application of concepts of quantity, unit and scale, no less than the definitions of those concepts (Riordan 2015; Tal 2016). The relationship between the definition and realizations of a unit becomes especially complex when the definition is stated in theoretical terms. Several of the base units of the International System (SI) — including the meter, kilogram, ampere, kelvin and mole — are no longer defined by reference to any specific kind of physical system, but by fixing the numerical value of a fundamental physical constant. The kilogram, for example, was redefined in 2019 as the unit of mass such that the numerical value of the Planck constant is exactly 6.62607015 × 10-34 kg m2 s-1 (BIPM 2019:131). Realizing the kilogram under this definition is a highly theory-laden task. The study of the practical realization of such units has shed new light on the evolving relationships between measurement and theory (Tal 2018; de Courtenay et al 2019; Wolff 2020b).