A hyperlink, or simply a link, is a reference in a hypertext document to another document or other resource. As such it would be similar to a citation in literature. However, combined with a data network and suitable access protocol, it can be used to fetch the resource referenced. This can then be saved, viewed, or displayed as part of the referencing document.

Hyperlinks are part of the foundation of the World Wide Web created by Tim Berners-Lee.

 

Hyperlinks in various technologies

 

Hyperlinks in HTML

Tim Berners-Lee saw the possibility of using hyperlinks to link every unit of information to any other unit of information over the Internet. Hyperlinks were therefore integral to the creation of the World Wide Web.

Links are specified in HTML using the <a> (anchor) elements.

 

Hyperlinks in XML

A special W3C Recommendation called the XML Linking Language, XLink, describes simple (i.e. as in HTML) and extended links for hyperlinking from, within, and between XML documents.

 

Hyperlinks in other technologies

Hyperlinks are used in PDF documents, word processing documents, spreadsheets, Apple's HyperCard and many others.

 

How hyperlinks work in HTML

A link has two ends, called anchors, and a direction. The link starts at the source anchor and points to the destination anchor. However, the term link is often used for the source anchor, while the destination anchor is called the link target.

The most common link target is a URL used in the World Wide Web. 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 a HTML element 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 "#attribute name" appended.

 

Link behaviour in web browsers

A web browser usually displays a hyperlink in some distinguishing way, e.g. in a different colour, font or style. The behaviour and style of links can be specified using the Cascading Style Sheets (CSS) language.

In a graphical user interface, the usage of a mouse cursor may also change into a hand motif to indicate a link. In most graphical browsers, links are displayed in underlined blue text when not cached, but underlined purple text when cached. When the user activates the link (e.g. by clicking on it with the mouse) the browser will display the target of the link. If the target is not a html-file, depending on the file type and on the browser and its plug-ins, another program may be activated to open the file.

The HTML code contains some or all of the four main characteristics of a link:

• link target (URL)

• link label

• link title

• link class or link id

It uses the HTML element "a" with the attribute "href" and optionally also the attributes "title", and "class" or "id":

<a href="URL" title="link title" class="link class">link label</a>

When the cursor hovers over a link, depending on the browser and/or graphical user interface, some informative text about the link is shown:

• It pops 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). IE and Mozilla Firefox show the title, Opera also shows the URL.

• In addition, the URL may be shown in the status bar. Opera and Mozilla Firefox give the full URL, IE gives it from the last slash, or if it ends with a slash, from the last but one.

 

Hyperlinks as the currency of the World Wide Web

The Google search engine uses PageRank, a measure of link popularity to determine which page should be ranked first. The more pages that have a hyperlink pointing to a page, the higher rank that page gets. It is actually slightly more complicated than that, see PageRank for more information.

 

History of the hyperlink

The term "hyperlink" was coined in 1965 (or possibly 1964) by Theodor Nelson at the start of Project Xanadu. Nelson had been inspired by "As We May Think," a popular essay by Vannevar Bush. In the essay, Bush described a microfilm-based machine 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. The closest contemporary analogy would be to build a list of bookmarks to topically related Web pages and then allow the user to scroll forward and backward through the list.

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 worldwide computer network, and advocated the creation of such a network. Meanwhile, working independently, a team led by Douglas Engelbart (with Jeff Rulifson as chief programmer) 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). See NLS.

 

Legal issues concerning hyperlinks

While hyperlinking among pages of Internet content has long been considered an intrinsic feature of the Internet medium, some websites have claimed that linking to them is not allowed without permission, see e.g. [1] (http://www.litmanlaw.com/content.aspx?page=243&section=12) and [2] (http://www.stib.irisnet.be/msgN.htm) (in Dutch). You do not need to ask permission to link to any page of Wikipedia's.

In some jurisdictions it is or was (for example the Netherlands, see Karin Spaink) held that hyperlinks are not merely references or citations, but are devices for copying web pages. Although this principle is generally rejected by digerati [3] (http://www.edge.org/digerati/), the courts that adhere to it see the mere publication of a hyperlink that connects to illegal material to be an illegal act in itself, regardless of whether referencing illegal material is illegal.

British Telecom sued Prodigy claiming that Prodigy infringed its patent ( U.S. Patent 4,873,662 (http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1=4,873,662.WKU.&OS=PN/4,873,662&RS=PN/4,873,662)) on web hyperlinks. However, after costly litigation, a court found for Prodigy, ruling that British Telecom's patent did not actually cover web hyperlinks. [1]

Deep linking, on the World Wide Web, is the process of placing on a web page a hyperlink that points to a specific page or image within another website, as opposed to that website's main or home page. Such links are called deep links.

Some commercial websites object to other sites making deep links into their content, either because it bypasses advertising on their main pages or, like The Wall Street Journal, they charge users for permanently-valid links. Sometimes deep linking has led to legal action, such as in the 1997 case of Ticketmaster versus Microsoft, where Microsoft deep-linked to Ticketmaster's site from its Sidewalk service. Many critics charge that such sites simply want to establish policies that will "license" such links to the highest bidder. They argue that links are a fundamental part of "user-oriented" web browsing.

The technology behind the World Wide Web, the Hypertext Transfer Protocol (HTTP), does not actually make any distinction between deep links and any other link—all links are conceptually equal. This is because the World Wide Web in itself does not have a concept of a "website." It is only because of the human user's need to group related web pages together that websites are conceptualized. Thus, the concept of deep linking—linking to pages other than the "home page" of the website.

Examples of deep linking to maps created on the fly can be found at the article Oakland, Illinois. The web addresses contain all the parameters of the maps. Sometimes this is not possible because a cache area (in this case, a shared script that generates an image map) is used to create the desired map through zooming and shifting, and there is no web address that directly gives the resulting map.

In HTML, inline linking is the placing of a linked object, often an image, from one site in a page belonging to a second site. The second site is said to have an inline link to the one where the object is located. It's used for such activities as linking images from personal home page storage into the online diary of the person controlling the personal home page.

This has sometimes been controversial because it is possible that the site where the object is stored and from which it is retrieved will not like the new placement or will consider it to be bandwidth theft.

Inline linking of images raises many ethical concerns. It may create legal problems, as well. In 2003 the legal status of inline linking had not been completely resolved and cases such as the US Kelly v. Arriba Soft Corporation were still resolving aspects of fair use and copyright infringement. In principle, a copyright infringement may occur when a browser combines material from several web sites into a single page image, and a copyright holder has not given permission for this kind of derived work. However if the website designer who made use of inline linking was to be targeted in legal action, a doctrine such as contributory copyright infringement would seem to be necessary, since they do not copy any material themselves from the other website, but just provide a link to it.

On a web page, framing means that the browser window is divided into frames, and that the HTML code for one frame contains a link to a web page on another website such that these external contents are automatically displayed within the frame (transclusion). This may be confusing and inconvenient to the user: he or she can get the impression that the information belongs to the same website; also, less than the full browser window is available and the address bar is less informative. Some websites request not to be used in this way on other websites; some discourage it by including a framekiller script in its pages. The framing website runs a risk of being blamed for external content that e.g. is or becomes inaccurate or objectionable. [1] (http://www.aardvark.co.nz/linking.htm)