In accord with the general principles adopted by the ISO (ISO 31), the CIPM recommends that the following rules be observed when using the SI prefixes:

1. Prefix symbols are printed in roman (upright) type with no space between the prefix symbol and the unit symbol.



2. The grouping formed by the prefix symbol attached to the unit symbol constitutes a new inseparable symbol (of a multiple or submultiple of the unit concerned) which can be raised to a positive or negative power and combined with other unit symbols to form compound unit symbols.

   Examples:
   1 cm³ = (10^–2 m)³ = 10^–6 m³
   1 µs^–1 = (10^–6 s)^–1 = 10⁶ s^–1
   1 V/cm = (1 V)/(10^–2 m) = 10² V/m
   1 cm^–1 = (10^–2 m)^–1 = 10² m^–1.

3. Compound prefixes, i.e. prefixes formed by the juxtaposition of two or more SI prefixes, are not used.

   Example: 1 nm      but not      1 mµm.

4. A prefix is never used in isolation.

   Example: 10⁶/m³      but not      M/m³.