Examples with prefixes and powers × = × = = . + = + = . = = = 0.05 m. = = = = = . 3 MW = = 3 × = . Micro symbol When mega and micro were adopted in 1873, there were then three prefaces starting with "m", so it was necessary to use some other symbol besides upper and lowercase 'm'. Eventually the Greek letter "µ" was adopted. However various other abbreviations remained common, including "mic", "mm", and "u", in part motivated by the lack of a "µ" key on most typewriters. From about 1960 onwards, "u" prevailed in type-written documents. Because ASCII, EBCDIC, and other common encodings lacked code-points for "µ", this tradition remained even as computers replaced typewriters. When ISO 8859-1 was created, it included the "" symbol for micro at codepoint . The whole of ISO 8859-1 was incorporated into the initial version of Unicode, but subsequently Unicode version 6 deprecated the micro symbol on codepoint in favour of the Greek letter "" on codepoint . However it remains widely implemented due to its prevalence in older documents, especially on POSIX systems where UTF-8 has historically been well tolerated by utilities that previously handled ASCII. Keyboard entry Most keyboards do not have a "µ" key, so it is necessary to use a key-chord; this varies depending on the operating system, physical keyboard layout, and user's language. For all keyboard layouts On Microsoft Windows systems, arbitrary Unicode codepoints can be entered in hexadecimal as: ; note that a leading "0" is required, or in the tradition of MS-DOS, IBM code page 437 code-points can be entered in decimal: (this is recorded as the corresponding Unicode code-point); On Linux systems, arbitrary Unicode codepoints can be entered in hexadecimal as: , or For QWERTY keyboard layouts On Linux systems, code-point U+00b5 can be entered as (provided the right alt key is configured to act as ). On MacOS systems, code-point U+00b5 can be entered as either or . Typesetting in Latex The LaTeX typesetting system features an SIunitx package in which the units of measurement are spelled out, for example, \SI{3}{\tera\hertz} formats as "3 THz". Application to units of measurement The use of prefixes can be traced back to the introduction of the metric system in the 1790s, long before the 1960 introduction of the SI. The prefixes, including those introduced after 1960, are