The kilogram is the standard unit for mass in SI and the metric system. The kilogram was originally defined as the mass of 1 cubic decimeter of pure water at 4° Celsius. A platinum cylinder of the same mass as the cubic decimeter of water was the standard until 1889. A platinum-iridium cylinder with the same mass is permanently kept near Paris. Copies exist in many countries. In the United States the National Institute of Standards and Technology (NIST) houses the mass standard, as well as the atomic clocks that define the second. The kilogram is the only standard unit that is not based on atoms or molecules. Several methods are under development to define the kilogram in terms of the mass of the carbon atom. Currently one method has a precision of 35 parts per billion. That is equivalent to measuring the mass of your body and the change in mass if one hair falls off your head!