The Eleventh General Conference of Weights and Measures recommended only the fundamental and derived units of the S.I system. But it didn’t elaborate on the rules for the usage of these units. Later on, many scientists and engineers held a number of meetings on the style and usage of S.I. units.
Rules for SI units:
1. A dash is to be used to separate units that are multiplied together. For example, a newton-meter is written as N-m. It should not be confused with mN, which stands for milli-newton.
2. For numbers having 5 or more digits, the digits should be placed in groups of three separated by spaces counting both to the left and right of the decimal point.
3. In a four-digit number, the space isn’t required unless the four-digit number is used in a column of numbers with 5 or more digits.