Now the units for almost all physical quantities (the few exceptions are those that are not dependent on time, length or force, e.g. angles and amount of substance) are derived from the unit of frequency.

The frequency of an atomic clock with cesium has a conventional value, and the units for the other physical quantities, e.g. length, force, voltage, electrical current, temperature and so on are derived from the unit of frequency by conversion factors that are functions of various universal constants, all of which have conventional values chosen so that the units derived in this way are approximately equal with the units that have been used in the past.