Spatially uniform birefringences tunable over the range 0. 0–0. 15 for applied voltages less than 20 V rms and with a sharp threshold below 4 V rms are obtained for thin layers of nematic liquid crystals with negative dielectric anisotropy. The achievement of spatially uniform tunable birefringence with negative‐dielectric‐anisotropy materials requires a source of in‐plane anisotropy which is provided by a unique previously unreported molecular configuration at zero field. AtV=0, the molecules align with their long axes at a small angle to the sample normal. The experimental results are analyzed in terms of a continuum model and experimental values obtained for the relevant elastic constants.