Cesium is used as a time keeper in atomic clocks. Atomic clock is a type of clock that uses certain resonance frequencies of atoms (usually caesium or rubidium) to keep time with extreme accuracy. The accuracy of an atomic clock depends on two factors. The first factor is temperature of the sample atoms—colder atoms move much more slowly, allowing longer probe times. The second factor is the frequency and intrinsic width of the electronic transition. Higher frequencies and narrow lines increase the precision.