 Structural properties have been invoked for many reasons. Armstrong (1978b: ch. 22) appeals to them to explain the resemblance of universals belonging to a common class, e.g., lengths and colors. Armstrong (1988; 1989) also treats physical quantities and numbers through structural properties. The former involve, as in the above mass 1 kg example, smaller quantities such as mass 0.1 kg and mass 0.2 kg (this view is criticized by Eddon 2007). The latter are internal proportion relations between structural properties (e.g., being a nineteen-electrons aggregate) and unit-properties (e.g., being an electron). Moreover, structural properties have been appealed to in treatments of laws of nature (Armstrong 1989; Lewis 1986a), some natural kinds (Armstrong 1978b, 1997; Hawley &amp; Bird 2011), possible worlds (Forrest 1986), ersatz times (Parsons 2005), emergent properties (O’Connor &amp; Wong 2005), linguistic types (Davis 2014), ontic structural realism (Psillos 2012). However, structural universals have also been questioned on various grounds, as we shall now see.