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computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...
, a hyperlink, or simply a link, is a digital reference providing direct access to
data Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...
by a user's clicking or tapping. A hyperlink points to a whole document or to a specific element within a document. Hypertext is text with hyperlinks. The text that is linked from is known as anchor text. A software system that is used for viewing and creating hypertext is a ''hypertext system'', and to create a hyperlink is ''to hyperlink'' (or simply ''to link''). A user following hyperlinks is said to ''navigate'' or ''browse'' the hypertext. The document containing a hyperlink is known as its source document. For example, in content from
Wikipedia Wikipedia is a free content, free Online content, online encyclopedia that is written and maintained by a community of volunteers, known as Wikipedians, through open collaboration and the wiki software MediaWiki. Founded by Jimmy Wales and La ...
or
Google Search Google Search (also known simply as Google or Google.com) is a search engine operated by Google. It allows users to search for information on the World Wide Web, Web by entering keywords or phrases. Google Search uses algorithms to analyze an ...
, many words and terms in the text are hyperlinked to definitions of those terms. Hyperlinks are often used to implement reference mechanisms such as tables of contents,
footnotes In publishing, a note is a brief text in which the author comments on the subject and themes of the book and names supporting citations. In the editorial production of books and documents, typographically, a note is usually several lines of t ...
, bibliographies,
indexes Index (: indexes or indices) may refer to: Arts, entertainment, and media Fictional entities * Index (A Certain Magical Index), Index (''A Certain Magical Index''), a character in the light novel series ''A Certain Magical Index'' * The Index, a ...
, and glossaries. In some hypertext, hyperlinks can be bidirectional: they can be followed in two directions, so both ends act as anchors and as targets. More complex arrangements exist, such as many-to-many links. The effect of following a hyperlink may vary with the hypertext system and may sometimes depend on the link itself; for instance, on the
World Wide Web The World Wide Web (WWW or simply the Web) is an information system that enables Content (media), content sharing over the Internet through user-friendly ways meant to appeal to users beyond Information technology, IT specialists and hobbyis ...
most hyperlinks cause the target document to replace the document being displayed, but some are marked to cause the target document to open in a new window (or, perhaps, in a new tab). Another possibility is transclusion, for which the link target is a document fragment that replaces the link anchor within the source document. Not only persons browsing the document may follow hyperlinks. These hyperlinks may also be followed automatically by programs. A program that traverses the hypertext, following each hyperlink and gathering all the retrieved documents is known as a Web ''spider'' or crawler.


Links


Inline links

An inline link displays remote content without the need for embedding the content. The remote content may be accessed with or without the user following the link. An inline link may display a modified version of the content; for instance, instead of an image, a thumbnail, low resolution preview, cropped section, or magnified section may be shown. The full content is then usually available on demand, as is the case with print publishing software e.g., with an external link. This allows for smaller file sizes and quicker response to changes when the full linked content is not needed, as is the case when rearranging a page layout.


Anchor links

An anchor hyperlink (anchor link) is a link bound to a portion of a document, which is often called a fragment. The fragment is generally a portion of text or a heading, though not necessarily. For instance, it may also be a ''hot area'' in an image ( image map in
HTML Hypertext Markup Language (HTML) is the standard markup language for documents designed to be displayed in a web browser. It defines the content and structure of web content. It is often assisted by technologies such as Cascading Style Sheets ( ...
), a designated, often irregular part of an image. Fragments are marked with ''anchors'' (in any of various ways), which is why a link to a fragment is called an anchor link (that is, a link to an anchor). For example, in
XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing data. It defines a set of rules for encoding electronic document, documents in a format that is both human-readable and Machine-r ...
, the element " provides anchoring capability (as long as the DTD or schema defines it), and in
wiki markup A wiki ( ) is a form of hypertext publication on the internet which is Collaborative editing, collaboratively edited and managed by its audience directly through a web browser. A typical wiki contains multiple pages that can either be edit ...
, is a typical example of implementing it. In
word processor A word processor (WP) is a device or computer program that provides for input, editing, formatting, and output of text, often with some additional features. Early word processors were stand-alone devices dedicated to the function, but current word ...
apps, anchors can be inserted where desired and may be called ''bookmarks''. In URLs, the hash character (#) precedes the name of the anchor for the fragment. One way to define a hot area in an image is by a list of coordinates that indicate its boundaries. For example, a political map of Africa may have each country hyperlinked to further information about that country. A separate invisible hot area interface allows for swapping skins or labels within the linked hot areas without repetitive embedding of links in the various skin elements. Text hyperlink. Hyperlink is embedded into a word or a phrase and makes this text clickable. Image hyperlink. Hyperlink is embedded into an image and makes this image clickable. Bookmark hyperlink. Hyperlink is embedded into a text or an image and takes visitors to another part of a web page. E-mail hyperlink. Hyperlink is embedded into e-mail address and allows visitors to send an e-mail message to this e-mail address.


Fat links

A fat link (also known as a "one-to-many" link, an "extended link" or a "multi-tailed link") is a hyperlink which leads to multiple endpoints; the link is a set-valued function.


Uses in various technologies


HTML

Tim Berners-Lee Sir Timothy John Berners-Lee (born 8 June 1955), also known as TimBL, is an English computer scientist best known as the inventor of the World Wide Web, the HTML markup language, the URL system, and HTTP. He is a professorial research fellow a ...
saw the possibility of using hyperlinks to link any information to any other information over the
Internet The Internet (or internet) is the Global network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...
. Hyperlinks were therefore integral to the creation of the
World Wide Web The World Wide Web (WWW or simply the Web) is an information system that enables Content (media), content sharing over the Internet through user-friendly ways meant to appeal to users beyond Information technology, IT specialists and hobbyis ...
. Web pages are written in the hypertext mark-up language
HTML Hypertext Markup Language (HTML) is the standard markup language for documents designed to be displayed in a web browser. It defines the content and structure of web content. It is often assisted by technologies such as Cascading Style Sheets ( ...
. This is what a hyperlink to the home page of the W3C organization could look like in HTML code: W3C organization website This HTML code consists of several tags: * The hyperlink starts with an anchor opening tag , and includes a hyperlink reference href="https://www.w3.org/" to the URL for the page. (The URL is enclosed in quotes.) * The URL is followed by >, marking the end of the anchor opening tag. * The words that follow identify what is being linked; this is the only part of the code that is ordinarily visible on the screen when the page is rendered, but when the cursor hovers over the link, many browsers display the target URL somewhere on the screen, such as in the lower left-hand corner. * Typically these words are underlined and colored (for example, blue for a link that has not yet been visited and purple for a link already visited). * The anchor closing tag () terminates the hyperlink code. * The tag can also consist of various attributes such as the "rel" attribute which specifies the relationship between the current document and linked document. Webgraph is a graph, formed from
web page A web page (or webpage) is a World Wide Web, Web document that is accessed in a web browser. A website typically consists of many web pages hyperlink, linked together under a common domain name. The term "web page" is therefore a metaphor of pap ...
s as vertices and hyperlinks, as directed edges.


XLink

The W3C recommendation called XLink describes hyperlinks that offer a far greater degree of functionality than those offered in HTML. These extended links can be ''multidirectional'', remove linking from, within, and between XML documents. It can also describe ''simple links'', which are unidirectional and therefore offer no more functionality than hyperlinks in HTML.


Permalinks

Permalinks are URLs that are intended to remain unchanged for many years into the future, yielding hyperlinks that are less susceptible to link rot. Permalinks are often rendered simply, that is, as friendly URLs, so as to be easy for people to type and remember. Permalinks are used in order to point and redirect readers to the same
Web page A web page (or webpage) is a World Wide Web, Web document that is accessed in a web browser. A website typically consists of many web pages hyperlink, linked together under a common domain name. The term "web page" is therefore a metaphor of pap ...
, blog post or any online digital media. The scientific literature is a place where link persistence is crucial to the public knowledge. A 2013 study in BMC Bioinformatics analyzed 15,000 links in abstracts from Thomson Reuters'
Web of Science The Web of Science (WoS; previously known as Web of Knowledge) is a paid-access platform that provides (typically via the internet) access to multiple databases that provide reference and citation data from academic journals, conference proceedi ...
citation index, founding that the median lifespan of Web pages was 9.3 years, and just 62% were archived. The median lifespan of a Web page constitutes high-degree variable, but its order of magnitude usually is of some months.


How hyperlinks work in HTML

A link from one domain to another is said to be ''outbound'' from its source anchor and ''inbound'' to its target. The most common destination anchor is a URL used in the
World Wide Web The World Wide Web (WWW or simply the Web) is an information system that enables Content (media), content sharing over the Internet through user-friendly ways meant to appeal to users beyond Information technology, IT specialists and hobbyis ...
. This can refer to a document, e.g. a webpage, or other resource, or to a position in a webpage. The latter is achieved by means of an
HTML element An HTML element is a type of HTML (HyperText Markup Language) document component, one of several types of HTML nodes (there are also text nodes, comment nodes and others). The first used version of HTML was written by Tim Berners-Lee in 199 ...
with a "name" or "id" attribute at that position of the HTML document. The URL of the position is the URL of the webpage with a fragment identifier "#''id attribute''" appended. When linking to PDF documents from an HTML page the "''id attribute''" can be replaced with syntax that references a page number or another element of the PDF, for example, "#''page=386''".


Link behavior in web browsers

A
web browser A web browser, often shortened to browser, is an application for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's scr ...
usually displays a hyperlink in some distinguishing way, e.g. in a different
color Color (or colour in English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is the visual perception based on the electromagnetic spectrum. Though co ...
,
font In metal typesetting, a font is a particular size, weight and style of a ''typeface'', defined as the set of fonts that share an overall design. For instance, the typeface Bauer Bodoni (shown in the figure) includes fonts " Roman" (or "regul ...
or
style Style, or styles may refer to: Film and television * ''Style'' (2001 film), a Hindi film starring Sharman Joshi, Riya Sen, Sahil Khan and Shilpi Mudgal * ''Style'' (2002 film), a Tamil drama film * ''Style'' (2004 film), a Burmese film * '' ...
, or with certain symbols following to visualize link target or document types. This is also called ''link decoration''. The behavior and style of links can be specified using the Cascading Style Sheets (CSS) language. In a graphical user interface, the appearance of a mouse cursor may change into a
hand A hand is a prehensile, multi-fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the Koala#Characteristics, koala (which has two thumb#O ...
motif to indicate a link. In most graphical web browsers, links are displayed in underlined blue text when they have not been visited, but underlined purple text when they have. When the user activates the link (e.g., by clicking on it with the
mouse A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus'' ...
) the browser displays the link's target. If the target is not an HTML file, depending on the file type and on the browser and its plugins, another program may be activated to open the file. The HTML code contains some or all of the five main characteristics of a link: * link destination ("href" pointing to a URL) * link label * link title * link target * link class or link id It uses the HTML element "a" with the attribute "href" (HREF is an abbreviation for "Hypertext REFerence") and optionally also the attributes "title", "target", and "
class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...
" or "id": :<a href="''URL''" title="''link title''" target="''link target''" class="''link class''">''link label''</a> To embed a link into a web page, blogpost, or comment, it may take this form: :Example In a typical web browser, this would display as the underlined word "Example" in blue, which when clicked would take the user to the example.com website. This contributes to a clean, easy to read text or document. By default, browsers will usually display hyperlinks as such: * An unvisited link is usually blue and underlined * A visited link is usually purple and underlined * An active link is usually red and underlined When the cursor hovers over a link, depending on the browser and graphical user interface, some informative text about the link can be shown, popping up, not in a regular window, but in a special hover box, which disappears when the cursor is moved away (sometimes it disappears anyway after a few seconds, and reappears when the cursor is moved away and back). Mozilla Firefox, IE,
Opera Opera is a form of History of theatre#European theatre, Western theatre in which music is a fundamental component and dramatic roles are taken by Singing, singers. Such a "work" (the literal translation of the Italian word "opera") is typically ...
, and many other web browsers all show the URL. In addition, the URL is commonly shown in the status bar. Normally, a link opens in the current frame or window, but sites that use frames and multiple windows for navigation can add a special "target" attribute to specify where the link loads. If no window exists with that name, a new window is created with the ID, which can be used to refer to the window later in the browsing session. Creation of new windows is probably the most common use of the "target" attribute. To prevent accidental reuse of a window, the special window names "_blank" and "_new" are usually available, and always cause a new window to be created. It is especially common to see this type of link when one large website links to an external page. The intention in that case is to ensure that the person browsing is aware that there is no endorsement of the site being linked to by the site that was linked from. However, the attribute is sometimes overused and can sometimes cause many windows to be created even while browsing a single site. Another special page name is "_top", which causes any frames in the current window to be cleared away so that browsing can continue in the full window.


History

The term "link" was coined in 1965 (or possibly 1964) by
Ted Nelson Theodor Holm Nelson (born June 17, 1937) is an American pioneer of information technology, philosopher, and sociologist. He coined the terms ''hypertext'' and ''hypermedia'' in 1963 and published them in 1965. According to his 1997 ''Forbes'' p ...
at the start of Project Xanadu. Nelson had been inspired by " As We May Think", a popular 1945 essay by Vannevar Bush. In the essay, Bush described a microfilm-based machine (the Memex) in which one could link any two pages of information into a "trail" of related information, and then scroll back and forth among pages in a trail as if they were on a single microfilm reel. In a series of books and articles published from 1964 through 1980, Nelson transposed Bush's concept of automated cross-referencing into the computer context, made it applicable to specific text strings rather than whole pages, generalized it from a local desk-sized machine to a theoretical proprietary worldwide computer network, and advocated the creation of such a network. Though Nelson's Xanadu Corporation was eventually funded by
Autodesk Autodesk, Inc. is an American multinational software corporation that provides software products and services for the architecture, engineering, construction, manufacturing, media, education, and entertainment industries. Autodesk is headquarte ...
in the 1980s, it never created this proprietary public-access network. Meanwhile, working independently, a team led by Douglas Engelbart (with Jeff Rulifson as chief
programmer A programmer, computer programmer or coder is an author of computer source code someone with skill in computer programming. The professional titles Software development, ''software developer'' and Software engineering, ''software engineer' ...
) was the first to implement the hyperlink concept for scrolling within a single document (1966), and soon after for connecting between paragraphs within separate documents (1968), with NLS. Ben Shneiderman working with graduate student Dan Ostroff designed and implemented the highlighted link in the HyperTIES system in 1983. HyperTIES was used to produce the world's first electronic journal, the July 1988
Communications of the ACM ''Communications of the ACM'' (''CACM'') is the monthly journal of the Association for Computing Machinery (ACM). History It was established in 1958, with Saul Rosen as its first managing editor. It is sent to all ACM members. Articles are i ...
, which was cited as the source for the link concept in
Tim Berners-Lee Sir Timothy John Berners-Lee (born 8 June 1955), also known as TimBL, is an English computer scientist best known as the inventor of the World Wide Web, the HTML markup language, the URL system, and HTTP. He is a professorial research fellow a ...
's Spring 1989 manifesto for the Web. In 1988, Ben Shneiderman and Greg Kearsley used HyperTIES to publish "Hypertext Hands-On!", the world's first electronic book. Released in 1987 for the Apple Macintosh, the database program
HyperCard HyperCard is a application software, software application and software development kit, development kit for Apple Macintosh and Apple IIGS computers. It is among the first successful hypermedia systems predating the World Wide Web. HyperCard com ...
allowed for hyperlinking between various pages within a document, as well as to other documents and separate applications on the same computer. In 1990, Windows Help, which was introduced with Microsoft Windows 3.0, had widespread use of hyperlinks to link different pages in a single help file together; in addition, it had a visually different kind of hyperlink that caused a popup help message to appear when clicked, usually to give definitions of terms introduced on the help page. The first widely used open protocol that included hyperlinks from any Internet site to any other Internet site was the
Gopher protocol The Gopher protocol () is a communication protocol designed for distributing, searching, and retrieving documents in Internet Protocol networks. The design of the Gopher protocol and user interface is menu-driven, and presented an alternative t ...
from 1991. It was soon eclipsed by HTML after the 1993 release of the Mosaic browser (which could handle Gopher links as well as HTML links). HTML's advantage was the ability to mix graphics, text, and hyperlinks, unlike Gopher, which just had menu-structured text and hyperlinks.


Legal issues

While hyperlinking among webpages is an intrinsic feature of the web, some websites object to being linked by other websites; some have claimed that linking to them is not allowed without permission. Contentious in particular are deep links, which do not point to a site's home page or other entry point designated by the site owner, but to content elsewhere, allowing the user to bypass the site's own designated flow, and ''inline links'', which incorporate the content in question into the pages of the linking site, making it seem part of the linking site's own content unless an explicit attribution is added. In certain
jurisdiction Jurisdiction (from Latin 'law' and 'speech' or 'declaration') is the legal term for the legal authority granted to a legal entity to enact justice. In federations like the United States, the concept of jurisdiction applies at multiple level ...
s, it is or has been held that hyperlinks are not merely
reference A reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''nam ...
s or citations, but are devices for copying web pages. In the Netherlands, Karin Spaink was initially convicted in this way of copyright infringement by linking, although this ruling was overturned in 2003. The courts that advocate this view see the mere
publication To publish is to make content available to the general public.Berne Convention, articl ...
of a hyperlink that connects to illegal material to be an illegal act in itself, regardless of whether referencing illegal material is illegal. In 2004, Josephine Ho was acquitted of 'hyperlinks that corrupt traditional values' in
Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia. The main geography of Taiwan, island of Taiwan, also known as ''Formosa'', lies between the East China Sea, East and South China Seas in the northwestern Pacific Ocea ...
. In 2000, British Telecom sued Prodigy, claiming that Prodigy infringed its patent () on web hyperlinks. After litigation, a
court A court is an institution, often a government entity, with the authority to adjudicate legal disputes between Party (law), parties and Administration of justice, administer justice in Civil law (common law), civil, Criminal law, criminal, an ...
found for Prodigy, ruling that British Telecom's patent did not cover web hyperlinks. In United States ''
jurisprudence Jurisprudence, also known as theory of law or philosophy of law, is the examination in a general perspective of what law is and what it ought to be. It investigates issues such as the definition of law; legal validity; legal norms and values ...
'', there is a distinction between the mere act of linking to someone else's website, and linking to content that is illegal (e.g., gambling illegal in the US) or infringing (e.g., illegal MP3 copies). Several courts have found that merely linking to someone else's website, even if by bypassing commercial advertising, is not copyright or trademark infringement, regardless of how much someone else might object. Linking to illegal or infringing content can be sufficiently problematic to give rise to legal liability. Compare for a summary of the current status of US copyright law as to hyperlinking, see the discussion regarding the ''Arriba Soft'' and ''Perfect 10'' cases. Somewhat controversially, Vuestar Technologies has tried to enforce
patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an sufficiency of disclosure, enabling discl ...
s applied for by its owner, Ronald Neville Langford,TelecomTV – TelecomTV One – News
around the world relating to search techniques using hyperlinked images to other
websites A website (also written as a web site) is any web page whose content is identified by a common domain name and is published on at least one web server. Websites are typically dedicated to a particular topic or purpose, such as news, education ...
or web pages.All your Interwibble is belong to us
Silvie Barak, The Inquirer, 21 February 2009


See also

*
Backlink From the point of view of a given web resource (referent), a backlink is a regular hyperlink on another web resource (the referrer) that points to the referent. A ''web resource'' may be (for example) a website, web page, or web directory. A ba ...
*
Dereference In computer science, a pointer is an object (computer science), object in many programming languages that stores a memory address. This can be that of another value located in computer memory, or in some cases, that of memory-mapped I/O, memo ...
(operator) * Internal link * Link building * Link rot * Object hyperlinking * Overlinking *
PageRank PageRank (PR) is an algorithm used by Google Search to rank web pages in their search engine results. It is named after both the term "web page" and co-founder Larry Page. PageRank is a way of measuring the importance of website pages. Accordin ...
* URI fragment * Xenu's Link Sleuth * Nofollow


References


Further reading

* {{Authority control Graphical control elements Hypertext