Context : Galactic dust emission is polarized at unexpectedly high levels , as revealed by Planck . Aims : The origin of the observed \simeq 20 \% polarization fractions can be identified by characterizing the properties of optical starlight polarization in a region with maximally polarized dust emission . Methods : We measure the R-band linear polarization of 22 stars in a region with a submillimeter polarization fraction of \simeq 20 \% . A subset of 6 stars is also measured in the B , V and I bands to investigate the wavelength dependence of polarization . Results : We find that starlight is polarized at correspondingly high levels . Through multiband polarimetry we find that the high polarization fractions are unlikely to arise from unusual dust properties , such as enhanced grain alignment . Instead , a favorable magnetic field geometry is the most likely explanation , and is supported by observational probes of the magnetic field morphology . The observed starlight polarization exceeds the classical upper limit of \left [ p _ { V } / E \left ( B - V \right ) \right ] _ { max } = 9 \% mag ^ { -1 } and is at least as high as 13 % mag ^ { -1 } that was inferred from a joint analysis of Planck data , starlight polarization and reddening measurements . Thus , we confirm that the intrinsic polarizing ability of dust grains at optical wavelengths has long been underestimated . Conclusions :