Editing Metric system (section)

=== Decimal ratios ===
A significant characteristic of the metric system is its use of decimal multiples {{endash}} powers of 10. For example, a length that is significantly longer or shorter than 1 metre can be represented in units that are a power of 10 or 1000 metres. This differs from many older systems in which the ratio of different units varied. For example, 12 [[inch]]es is one [[foot (unit)|foot]], but the larger unit in the same system, the [[mile]] is not a power of 12 feet. It is 5,280 feet {{endash}} which is hard to remember for many.<ref name=":0" />{{Rp|page=17}}

In the early days, multipliers that were positive powers of ten were given Greek-derived prefixes such as  ''kilo-'' and ''mega-'', and those that were negative powers of ten were given Latin-derived prefixes such as ''centi-'' and ''milli-''. However, 1935 extensions to the prefix system did not follow this convention: the prefixes ''nano-'' and ''micro-'', for example have Greek roots.<ref name="McGreevy v2">{{cite book |title = The Basis of Measurement: Volume 2—Metrication and Current Practice |isbn = 978-0-948251-84-9 |publisher = Picton Publishing |location = Chippenham |year = 1997 |first1 = Thomas |last1 = McGreevy |editor1-first = Peter |editor1-last = Cunningham}}</ref>{{rp|222–223}} During the 19th century the prefix [[Non-SI unit prefix#Obsolete prefixes|''myria-'']], derived from the Greek word μύριοι (''mýrioi''), was used as a multiplier for {{val|10000}}.<ref>{{cite book |url=https://archive.org/details/edinburghencyclo07brew |title=The Edinburgh Encyclopædia |first1=D. |last1=Brewster |page=[https://archive.org/details/edinburghencyclo07brew/page/494 494] |year=1830 }}</ref>

When applying prefixes to derived units of area and volume that are expressed in terms of units of length squared or cubed, the square and cube operators are applied to the unit of length including the prefix, as illustrated below.<ref name=SI_prefix />
{| style="margin-left:3em !important; white-space:nowrap"
|-
|1&nbsp;mm<sup>2</sup> (square millimetre)         ||= (1&nbsp;mm)<sup>2</sup>&nbsp;||= (0.001&nbsp;m)<sup>2</sup>&nbsp;||= {{val|0.000001|u=m2}}
|-
|1&nbsp;km<sup>2</sup> ([[square kilometre]])&nbsp;||= (1&nbsp;km)<sup>2</sup>      ||= (1000&nbsp;m)<sup>2</sup>       ||= {{val|1000000|u=m2}}
|-
|1&nbsp;mm<sup>3</sup> (cubic millimetre)          ||= (1&nbsp;mm)<sup>3</sup>      ||= (0.001&nbsp;m)<sup>3</sup>      ||= {{val|0.000000001|u=m3}}
|-
|1&nbsp;km<sup>3</sup> (cubic kilometre)           ||= (1&nbsp;km)<sup>3</sup>      ||= (1000&nbsp;m)<sup>3</sup>       ||= {{val|1000000000|u=m3}}
|}

For the most part, the metric prefixes are used uniformly for SI base, derived and accepted units. A notable exception is that for a large measure of seconds, the non-SI units of [[minute]], [[hour]] and [[day]] are customary instead. Units of duration longer than a day are problematic since both month and year have varying number of days. Sub-second measures are often indicated via submultiple prefixes. For example, [[millisecond]].<ref name=SI_prefix />