Help:Interwiki linking
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General
Within each project, interwiki linking to a page of another project is possible with internal link style, with a prefix for the other project ("prefixed internal link style"). For each project, an interwiki map (or "InterMap") is specified, i.e. a list of target projects with their prefixes, e.g. [1]. These target projects need not use MediaWiki and need not even be a wiki.
For a given project the namespace prefixes can not also be used as code for an external project. However, the prefix used for a target project may coincide with the prefix for the project namespace within that project. As a result, to link to a page in that namespace one has to use the same prefix twice, e.g. wikibooks:wikibooks:Manual of Style.
For a multilingual family of similar projects, with one project per language, a system for interlanguage linking can be set up.
An interwiki link within the family is treated differently unless it is on a talk page of any namespace: it appears at one or two edges of the webpage (in Monobook on the left, in Classic at the top and bottom); to make it inline, prefix a colon.
The link label depends only on the sister project that is linked to, not on the page; it is set in the configuration of the project, typically it is the name of the language in that language. The target is only shown in the hover box and the status bar, depending on the browser.
Therefore this kind of link is mainly suitable for linking to the corresponding page in another language, and not very suitable for multiple links to the same other language. See also Interlanguage use case for a discussion of common troubles with this system and other possible implementations.
The feature can also be used on an image description page, to link to the same or a similar image in a sister project. Other interwiki links to images require the prefixed colon.
Note that a page that might be used as a template, even if it is in an other than the template namespace, is not very suitable to put an interlanguage link in: such a link appears in the edge of the page that includes the template, giving the impression that the link is to a version in the other language of the referring page instead of the template. For the same reason pages in the MediaWiki namespace are not very suitable to put an interlanguage link in.