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Because [[SI prefix]]es may not be concatenated (serially linked) within the name or symbol for a unit of measure, SI prefixes are used with the ''[[gram]]'', not the kilogram, which already has a prefix as part of its name.<ref>BIPM: SI Brochure: Section 3.2, ''[http://www.bipm.org/en/si/si_brochure/chapter3/3-2.html The kilogram]<sub>{{nbsp}}</sub></ref> For instance, one-millionth of a kilogram is 1{{nbsp}}mg (one milligram), not 1{{nbsp}}µkg (one microkilogram).