<P> A unit of length refers to any discrete, pre-established length or distance having a constant magnitude which is used as a reference or convention to express linear dimension . The most common units in modern use are U.S. customary units in the United States and metric units elsewhere . British Imperial units are still used for some purposes in the United Kingdom and some other countries . The metric system is sub-divided into SI and non-SI units . </P> <P> The base unit in the International System of Units (SI) is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of ​ ⁄ seconds ." It is approximately equal to 6999999987839999999 ♠ 1.0936 yd . Other units are derived from the metre by adding prefixes from the table below: </P> <Ul> <Li> v </Li> <Li> t </Li> <Li> e </Li> </Ul> <Table> Standard prefixes for the SI units of measure <Tr> <Th> Multiples </Th> <Th> Prefix name </Th> <Th> </Th> <Th> deca </Th> <Th> hecto </Th> <Th> kilo </Th> <Th> mega </Th> <Th> giga </Th> <Th> tera </Th> <Th> peta </Th> <Th> exa </Th> <Th> zetta </Th> <Th> yotta </Th> </Tr> <Tr> <Th> Prefix symbol </Th> <Td> </Td> <Td> da </Td> <Td> h </Td> <Td> k </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> Z </Td> <Td> Y </Td> </Tr> <Tr> <Th> Factor </Th> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> </Tr> <Tr> <Th_colspan="12"> </Th> </Tr> <Tr> <Th> Fractions </Th> <Th> Prefix name </Th> <Th> </Th> <Th> deci </Th> <Th> centi </Th> <Th> milli </Th> <Th> micro </Th> <Th> nano </Th> <Th> pico </Th> <Th> femto </Th> <Th> atto </Th> <Th> zepto </Th> <Th> yocto </Th> </Tr> <Tr> <Th> Prefix symbol </Th> <Td> </Td> <Td> d </Td> <Td> </Td> <Td> m </Td> <Td> μ </Td> <Td> n </Td> <Td> p </Td> <Td> f </Td> <Td> </Td> <Td> z </Td> <Td> y </Td> </Tr> <Tr> <Th> Factor </Th> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> <Td> 10 </Td> </Tr> </Table>

Where is the sample of unit length kept at