Ferroelectricity has been uncovered in Rochelle Salt (RS) by Valasek in 1920 (1). RS is a complicated crystal with four molecules [NaKC4 H4 06 , 4H2 0] in the primitive unit cell (2). Several external lattice vibrations have been observed in the 10 - 100 cm-1 far infrared range at 80 K and 5 K in crystal plates of increasing thickness : 160-550 and 6,500 μm. At 80 K absorption increases with thickness according to Beer's law. At 5 K, the lower frequency absorption line (21 cm-1) seems independent of thickness. This line had been shown to shift to a lower frequency at T = 130 K, and to disappear from the FIR range at a still higher temperature (3). More recently (4) it has been ascribed to the soft mode responsible for the FE transition with a resonant behaviour at 5 K and 80 K. It is softening into a relaxation mode whose frequency goes to zero when temperature increases up to the lower Curie temperature (T1 = 255 K). At 5 K the soft mode is a very weakly damped one, and we have to take into account the instrumental function for the discussion of the effect of plate thickness.