Editing Unit

[[File:Keycap sizes -- assorted units.jpg|thumb|250px|Assorted keycap sizes; clockwise from top-left: 1 unit, 1.75 unit, 1.25 unit, 1.5 unit]]

A '''unit''' is a measurement of keyboard key width, based on the size of a normal letter key (which is 1 unit tall by 1 unit wide).

==Details==
One unit is the distance from one key to the next; keycaps are fractionally less than their unit size to allow for gaps between keys. The spacing between single-unit keys is referred to as the '''key pitch''' or pitch of the keyboard.

Key sizes are usually in multiples of ½ (0.5) or ¼ (0.25) unit. For instance, a typical [[space bar]] for a modern PC keyboard is 6.25 units wide, taking up the same space as six and one quarter one-unit keys.

[[File:Unit.svg|center]]

Note that one unit of height may be different from one unit of width on a keyboard.

==Standards==
[[File:Keyb-1u-standard.svg|right]]
The key spacing unit is exactly defined as the distance between the centres of neighbouring letter keys in the vertical and in the horizontal projection. Normally, but not necessarily, the vertical and the horizontal unit have the same value. The unit value of today has been inherited from American typewriter keyboards of the late 19th century, which happened to have unit values of about ¾ inch. Owing to the market dominance of the American makes, this value made it into European national standards from the 1920s on.<ref>German standard DIN&nbsp;2111:1927, ''Tastenabstände – Richtlinien für Neukonstruktionen'', which has vertical and horizontal units of 19&nbsp;mm ± 1&nbsp;mm.</ref>
Whilst almost all full-size keyboards have unit values of about 19&nbsp;mm or ¾&nbsp;inch, there currently is no international standard that specifies it. There used to be ISO&nbsp;1091,<ref>ISO&nbsp;1091:1977, ''Typewriters – Layout of printing and function keys.'' There was a precursor, ISO/R&nbsp;1091, since 1967.</ref> which prescribed unit values of 19&nbsp;mm ± 1&nbsp;mm or 0.75" ± 0.04" horizontally and vertically; but this specification was lost in 1994 when the standard was withdrawn in favour of the ISO&nbsp;9995 series<ref>ISO/IEC 9995:1994, ''Information technology - Keyboard layouts for text and office systems''</ref>. Today, national standards have been reduced to what is specific to their national layout and otherwise refer to ISO&nbsp;9995. But since the unit definition is still missing in ISO&nbsp;9995, German DIN, for instance, keeps it in a preliminary standard, DIN&nbsp;2137-2,<ref>DIN&nbsp;2137-2:2012, ''Tastaturen für die Daten- und Texteingabe - Zusätzliche Anforderungen''. (Earlier, the unit was defined in separate standards DIN&nbsp;2112, DIN&nbsp;2127 and DIN&nbsp;2139 for mechanical typewriters, electric typewriters and computer keyboards, respectively.)</ref> while waiting for this to be ruled by an international standard.

Note that the tolerances do not add. Thus, if the standard sets a unit value of 19&nbsp;mm ± 1&nbsp;mm, the distance between the centres of 'Q' and 'P', which are 9&nbsp;units apart, has to be 171&nbsp;mm ± ''1''&nbsp;mm, not 171&nbsp;mm ± ''9''&nbsp;mm.

==Exceptions==
Some (not all) models of the [[Cherry G84 series]] with [[Cherry ML]] switches allegedly have a reduced unit size of about 18&nbsp;mm; this has, for instance, been measured for a G84-4400, but not for G84-90218, which has a 19&nbsp;mm unit size.

==Gallery==
<gallery widths=250 heights=187>
File:Keycap sizes -- 1.25 unit.jpg | 2 × 1 unit, 1 × 1.25 unit
File:Keycap sizes -- 1.5 unit.jpg  | 2 × 1 unit, 1 × 1.5 unit
File:Keycap sizes -- 1.75 unit.jpg | 2 × 1 unit, 1 × 1.75 unit
File:Keycap sizes -- 2 unit.jpg    | 2 × 1 unit, 1 × 2 unit
</gallery>

==See also==
*[[Keycap size by keyboard]]

==External links==
*[http://www.ergonomicsmadeeasy.com/keyboard-keypitch/ Ergonomics Made Easy] — 'keypitch' definition

==References==
<references>
</references>

[[Category:Keyboard terms]]