A typed link in a hypertext system is a link to another document or part of a document that includes information about the character of the link. For example, rather than merely pointing to the existence of a document, a link might also specify that the document supports the conclusion of the article pointing to it, that it contradicts the article pointing to it, that it is an older version of the document, that it serves to define the word next to the link, that it is an index to other documents of the same type, or some other relationship. This allows a user to take actions such as searching only certain types of links or displaying them differently. It may also allow browsing software to do things like pre-fetching documents it expects the user to browse.
While not an important part of Internet-based hypertext systems, typed links were a common feature in pre-Internet hypertext systems such as Xanadu, NoteCards, HyperWriter, IBIS/gIBIS, and others. Although they can be very useful, the lack of a standardized set of link attributes such as "Supports Position"/"Refutes Position" as well as the difficulty of applying the attributes has always hindered the use of typed links beyond prototyping purposes. Any system of attributes would turn a hypertext corpus into a semantic web, and permit more sophisticated processing.
Version 4 of the HTML standard defined by the World Wide Web Consortium supports typed links using the
rel (forward relationship) and
rev (reverse relationship) attributes. These attributes are applied to either the
<link> tag (for links between whole documents) or the
<a> tag (for links from a specific part of a document). For example, the tag
<link rel="contents" href="top.html"> specifies that the document "top.html" is a table of contents for the work that includes the document you are currently reading, and the tag
<link rel="next" href="chap3.html"> specifies that "chap3.html" is the next document in logical sequence after the one you are reading.
The following link types are defined in the HTML 4 standard:
Unofficial link types also exist. For example, Google has proposed to reduce the effect of comment spam in blogs with "rel=nofollow".
A semantic link is a typed link where the element itself includes meaningful information about the link (semantics). The link describes some external relationship or issue. Such links are the basis of a semantic network.
- In XML, a BookTitle element such as <BookTitle isbn="0812511816" />. You can use the semantic information provided in the element (the ISBN) to create a link from this element to an online bookseller.
- Any syntax that can express genealogy, such as "A is-mother-of B". Other relations like "B is-child-of A" may be implied by it.
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