Changes

The SI system introduced several new units for physical terms and continues to change to this day. As measurement accuracy improves, so the standards against which measures are calibrated must also be defined to higher accuracy. Definitions have been subject to change through the years, and generally depend on assumptions that the CGPM makes about what is permanent and what is not.

The SI system introduced several new units for physical terms and continues to change to this day. As measurement accuracy improves, so the standards against which measures are calibrated must also be defined to higher accuracy. Definitions have been subject to change through the years, and generally depend on assumptions that the CGPM makes about what is permanent and what is not.

Nearly all developed countries have adopted the SI system of units as official. The [[United States of America]] also uses the [[US customary system]], which is defined in terms of metric units since the Convention on the Meter.  

Nearly all countries have adopted the SI system of units as their official system of weights and measures. The [[United States of America]] also uses the [[US customary system]], which is defined in terms of metric units since the Convention on the Meter.  

==Principles==

==Principles==