21. Historically the " foot " was a part of many local systems of units , including the digits. 22. The manual then describes a system of unit identification that sounds like a fore-runner of medieval heraldry. 23. The base unit of energy within the International System of Units ( SI ) is the joule. 24. A standard system of units facilitates this. 25. SI is the official system of units , but traditional units are still ubiquitously used in everyday life. 26. Metric systems of units have evolved since the adoption of the first well-defined system in France in 1795. 27. The standard International System of Units ( SI ) unit of mass is the kilogram ( kg ). 28. Despite its name, IU is not part of the International System of Units used in physics and chemistry. 29. Finally, one more unit is needed to construct a usable system of units that includes energy and mass. 30. The standards-based unit in the International System of Units ( SI ) is the joule ( J ).
