==Writing Metric==
The rules of the SI system are defined in the [[International Organization for Standardization]] documents [[ISO 1000]] and [[ISO 31]]. They define not only the units, but the ways in which they are to be written.
* A space is to be put between the number and the unit. '25 km' is correct, '25 km25km' is incorrect. This is a common mistake for users of English units, where no space is placed between the number and the unit.
* SI units do not take a trailing 's' to denote the plural. Thus '5 cms' is wrong and '5 cm' is correct. The reason is that 's' is the symbol for the second and 'cms' (intended to mean centimeters) could be confused with 'cm s' (meaning centimeter-seconds, a derived unit)
* SI symbols do not take a trailing period. (This would be appropriate if they were abbreviations, but they are not.)