Polarised emission of the diffuse ISM between 0.2 and 3 mm normalized to the polarisation at 850 µm. Observations (circles) with 1σ error bars are shown in blue by Planck Collaboration XXI (2015); Planck Collaboration XII (2020), in magenta by Gandilo et al. (2016), in brown by Shariff et al. (2019), and in orange by Ashton et al. (2018), as tabulated by Hensley & Draine (2021). The model with µm (Table 1) and m_Dark = 6% of mass in dark dust (full line), the contributions from large amorphous carbon (brown) and large silicate grains (green), and the total polarisation fraction adopting m_Dark = 12% (dashed) is shown. The contribution from dark dust when it is treated as prolate particles is indicated by the area in grey.