Edingtonite is a ferroelectric material of the Rochelle salt (RS) class and shows large anomalies of different properties in the temperature range of -53 to -133 °C. Classical RS exhibits the low-temperature phase transition P2₁2₁2-P2 ₁11, whereas ammonium RS shows the phase transition P2 ₁2₁2-P1₁2₁1. According to our data, edingtonite blocks this possible symmetry lowering by the phase transition P2₁2₁2-P112. Edingtonite, similarly to RS crystals, has two sub-lattices of water molecules and exhibits a complex low-temperature behavior. There are at least three anomalies in the low-temperature dependence of the generation of the second optical 0.53-micron harmonics, excited by the 1.06-micron line of a Nd pulse laser. Edingtonite also exhibits significant anomalies in the low-frequency range of its IR spectrum at low temperatures. The maximum IR anomaly is observed in natural edingtonite at -124 °C, whereas in the deuterated sample the same anomaly occurs at -113 °C. The most intense anomalous broad band at 220 cm^-1 did not shift due to deuteration. According to the presented lattice-dynamical calculations, this vibration corresponds to a strong deformation of the cavity around the water molecules during the phase transition. Raman study of edingtonite under high pressure in a diamond anvil cell (DAC) at room temperature shows that there is no pressure-induced phase transition up to 6.4 GPa. No amorphization was observed at this pressure range.