The most accurate primary frequency standards in the world today reside in the national standards laboratories of several countries: They are an atomic‐fountain clock at the Paris Observatory's Bureau National de Me´trologie—Laboratoire Primaire de Temps et Fre´quences and two atomic‐beam clocks, one run by the US National Institute of Standards and Technology (NIST) in Boulder, Colorado, and another by the German Federal Institute of Physics and Metrology in Braunschweig.