 Physical quantities such as mass or length are typically treated as properties, specifiable in terms of a magnitude, a certain number, and a unit measure, e.g., kg or meter, which can itself be specified in terms of properties (Mundy 1987; Swoyer 1987). Following Eddon’s (2013) overview, it is possible to distinguish two main strands here: relational and monadic properties theories (see also Dasgupta 2013). In relational theories (Bigelow &amp; Pargetter 1988; 1991), quantities arise from proportion relations, which may in turn be related by higher-order proportion relations. Consider a certain quantity, e.g., mass 3 kg. Here the thrice as massive as proportion relation is relevant, which holds of certain pairs of objects \(a\) and \(b\) such that \(a\) is thrice as massive as \(b\). By virtue of such relational facts, the first members of such pairs have a mass of 3 kg. Another relational approach is by Mundy (1988), who claims that mass 3 kg is a relation between (ordered pairs of) objects and numbers. For example, mass 3 kg is a relation holding between the ordered pair \(\langle a, b\rangle\) (assuming that \(a\) is thrice as massive as \(b\), and not the other way round) and the number 3; or—if relations have built-in relational order—between \(a, b\) and 3. Knowles (2015) holds a similar view, wherein physical quantities are relations that objects bear to numbers. According to the monadic properties approach, quantities are intrinsic properties of objects (Swoyer 1987; Armstrong 1988; 1989). As we saw, Armstrong develops such a view by taking quantities to be structural properties. Related metaphysical inquiries concern the dimensions of quantities (Skow 2017) and the status of forces (Massin 2009).