High quality single BCPS crystals were used to study the influence of a structurally incommensurate (IC) modulation on the line shape in 35 Cl NMR spectra where the phase transition from the normal into the IC phase at ca. 150 K is clearly reflected in the appearance of an asymmetric frequency distribution with edge singularities. The line shape calculations are performed in a temperature range where the validity of the plane wave limit may be assumed. In nice agreement with an X-ray analysis of the rotational symmetry modes in the IC phase by Zuniga et al., the evaluation of the 35 Cl NMR frequency distributions is based on the IC modulation of the Euler angles which describe the orientation of 35 Cl NMR quadrupole coupling tensor in the crystal frame.