Two crystals of the monoclinic sphenoidal class have been found which have modes of vibration with zero temperature coefficients of frequency, and this property together with the high electromechanical coupling and the high $Q'\mathrm{s}$ make it appear probable that these crystals may have considerable use as a substitute for quartz which is difficult to obtain in large sizes. These crystals are ethylene diamine tartrate ${(\mathrm{C}}_{6}$${\mathrm{H}}_{14}$${\mathrm{N}}_{2}$${\mathrm{O}}_{6}$) and dipotassium tartrate ${(\mathrm{K}}_{2}$${\mathrm{C}}_{4}$${\mathrm{H}}_{4}$${\mathrm{O}}_{6}$, \textonehalf{}${\mathrm{H}}_{2}$O). Complete measurements of the elastic, piezoelectric, and dielectric constants of the dipotassium tartrate (DKT) crystal are given in this paper. The crystal has 4 dielectric constants, 8 piezoelectric constants, and 13 elastic constants. A discussion is given in the appendix of the method of measuring these constants by the use of 18 properly oriented crystals.