Two crystals of the monoclinic sphenoidal class have been found which have modes of vibration with zero temperature coefficients of frequency, high electromechanical coupling constants, and high Q's or low dissipation. These properties make it appear probable that such crystals may have a considerable use in filters and oscillators as a substitute for quartz, which is difficult to obtain in large sizes. These crystals are ethylene diamine tartrate (EDT) having the chemical formula C6H14N2O6, and di-potassium tartrate (DKT) having the formula K2C4H4O6-1/2 H2O. The paper describes the properties of EDT, since this crystal has been found more advantageous than DKT. The 13 elastic constants, the 8 piezoelectric constants, and the 4 dielectric constants have been measured over a temperature range, and from these measurements the regions of low temperature coefficients and high electromechanical coupling have been located. Six low-temperature-coefficient cuts have been discovered and the properties of these cuts are given. These cuts are being applied in the crystal channel filters of the long-distance telephone system, and may be applied to the control of oscillators.