A primary frequency standard in which electronic transitions between the two hyperfine ground states of cesium-133 atoms control the output frequency. Note 1: The difference between the energy levels of the two hyperfine ground states of the cesium-133 atom corresponds, in the absence of external influences (e.g., the magnetic field of the Earth,) to electromagnetic radiation having a frequency of precisely 9,192,631,770 Hz (microwave regime. ) Note 2: The preceding is exact because by internationally recognized (SI) definition, the second is the interval required for 9,192,631,770 cycles of this radiation to occur in the absence of external influences. A very slightly higher, but known and controlled, transition rate occurs in realizeable cesium standards, making the second by far the most precisely determined SI base unit.