The capability of a twisted nematic liquid crystal display to generate a set of equi-azimuth polarization states is used to achieve a phase-only modulation regime. For this purpose, a liquid crystal display followed by a quarter-wave plate is launched between two polarizers. Theoretical support is provided by means of the Jones matrix calculus and the Poincaré sphere representation. Laboratory results for a commercial liquid crystal display are presented. A phase modulation deep of 270 masculine is obtained at 514 nm with a residual intensity variation which is lower than 2.5 %.