 Because important physical quantities, such as spatial pressure of the cosmological fluid, are proportional to such time derivatives, the physical interpretation of sudden singularities is often, in at least one sense, perspicuous: depending on the time derivative that diverges, a sudden singularity can mark the divergence of a physically important quantity such as pressure, within a finite interval of proper time (Cattoën and Visser 2005; Cotsakis and Klaoudatou 2005; Fernández-Jambrina 2014; Jiménez et al. 2016). In such cases, it may happen that the mass-density of the fluid itself, the expansion factor and its first derivative, and even the Hubble parameter and its first derivative, all remain finite: only the pressure (and so the second derivative of the expansion factor) diverges. Because the physical significance of quantities such as pressure is thought to be unambiguous, this feature of sudden singularities stands in marked contrast to the problems of physical interpretation that plague the standard type of singularity, discussed in  section 1.3.  