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Nematic–nematic transitions in a highly polar nematic compound are studied, in thick cells in which the molecules are aligned parallel to the substrates but perpendicular to the applied electric field, using dielectric spectroscopy in the frequency range 1 Hz to 10 MHz over a wide temperature range. The studied compound displays three nematic phases under cooling from the isotropic phase: ubiquitous nematic N; high polarizability N_X; and ferroelectric nematic N_F. Two collective processes were observed. The dielectric strength and relaxation frequency of one of the processes P₂ showed a dependence on the thickness of the cell. The process P₁ is the amplitude mode, while the process P₂ is the phason mode.