A clock in which the intervals of time are determined by reference to the frequency at which a crystalline solid (crystal) oscillates when held between the plates of a condenser in a resonant circuit. The crystal is actually a flat plate cut from a quartz crystal at such an angle to the crystal's axes as to give the least variation in frequency of oscillation. The crystal is usually cut to give a frequency of 1 Mc/s or 5 Mc/s at resonance, although other frequencies that relate better to the sidereal day or the mean solar day have been used. The resonant frequency of the quartz plate is divided down to one cycle or pulse per second by electronic circuits. A good crystal clock has a long term stability of about 1 part in 1012.