Maximum linear polarisation fraction as a function of wavelength for a dust grain mixture consisting of partially aligned spheroidal silicate grains with a CDE2 shape distribution, and non-aligned spherical graphite grains. The polarisation fraction has been averaged out over an MRN size distribution. The different lines correspond to different grain mixtures: solid lines represent silicate-only mixtures where the alignment fraction f_S, A varies with the colour of the line; dashed lines represent a linear mixture of perfectly aligned silicate grains and non-aligned graphite grains.