Schematic representation of the polarization vectors from polarized emission by prolate (b/a = 0.9) aligned grains that would be observed in the three ALMA bands for a protoplanetary disk inclined by an angle of 45^° with a perfectly azimuthal magnetic field (dashed orange ellipses), for three values of the grain size corresponding to the peak of log-normal size distribution. The length of the vectors scales with the polarization fraction, reproducing the trends predicted by our model. Dotted lines indicate a polarization fraction that is close to zero. The contribution of self-scattering to the observed polarization is ignored.