Hyperlink

In computing, a hyperlink (or link) is a reference to data that the reader can directly follow, or that is followed automatically. A hyperlink points to a whole document or to a specific element within a document. Hypertext is text with hyperlinks. A software system 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.

A hyperlink has an anchor, which is the location within a document from which the hyperlink can be followed; the document containing a hyperlink is known as its source document. For example, in an online reference work such as Wikipedia, 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, bibliographies, indexes, letters 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, 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. 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 follow hyperlinks; they 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.

Types of links

Inline link

An CHIMICHANGAS inline link displays remote content without the need for embedding the content. The remote content may be accessed with or without the user selecting the link. For example, the image above is a document that can be viewed separately, but it is included into this page with an inline 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 will then usually be 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

An anchor hyperlink is a link bound to a portion of a document—generally text, though not necessarily. For instance, it may also be a hot area in an image (image map in HTML), a designated, often irregular part of an image. One way to define it 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.

Hyperlinks in various technologies

Hyperlinks in HTML

Tim Berners-Lee saw the possibility of using hyperlinks to link any information to any other information over the Internet. Hyperlinks were therefore integral to the creation of the World Wide Web. Web pages are written in the hypertext mark-up system HTML.

Links are specified in HTML using the <a> (anchor) elements. To see the HTML used to create a page, most browsers offer a "view page source" option. Included in the HTML code will be an expression in the form symbol "<a" and the reference "href="URL">" marking the start of an anchor, followed by the highlighted text and the "</a>" symbol, which indicates the end of the source anchor. The <a> element can also be used to indicate the target of a link.

Webgraph is a graph, formed from web pages as vertices and hyperlinks, as directed edges.

XLink: hyperlinks in XML

Main article: 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, linking from, within, and between XML documents. It also describes simple links, which are unidirectional and therefore offer no more functionality than hyperlinks in HTML.

Hyperlinks in other document technologies

Hyperlinks are used in the Gopher protocol, text editors, PDF documents, help systems such as Windows Help, word processing documents, spreadsheets, Apple's HyperCard and many other places.

Hyperlinks in virtual worlds

Main article: Hyperlinks in virtual worlds

Hyperlinks are being implemented in various 3D virtual world networks, including those which utilize the OpenSimulator and Open Cobalt platforms.

Hyperlinks in wikis

While wikis may use HTML-type hyperlinks, lightweight markup languages of wikis (wiki markup) provide simplified syntax, called wikilinks for linking pages within wiki environments.

The syntax and appearance of wikilinks may vary. Ward Cunningham's original wiki software, the WikiWikiWeb, used CamelCase for this purpose. CamelCase was also used in the early version of Wikipedia and is still used in some wikis, such as TiddlyWiki, Trac and PMWiki. A common markup is using double square brackets around the term to be wikilinked, for example, the following input: [[wiki software]] — will be converted by wiki software to look like this: wiki software.

Hyperlinks used in wikis are commonly classified as follows:

• Internal wikilinks or intrawiki links lead to pages within the same wiki website.
• Interwiki links are simplified markup hyperlinks lead to pages of other wikis.
• External links lead to other webpages.

If an internal wikilink leads to a page that does not exist, it usually has a distinct visual appearance. For example, in Wikipedia they are commonly displayed in red color, like this, and therefore they are called red links in Wikipedia. Another way is to display a highlighted clickable question mark by the wikilinked term, like this?.

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. 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 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 — "#attribute name" — appended.

When linking to PDF documents from an HTML page the "attribute name" can be replaced with syntax that references a page number or another element of the PDF, for example, page=[pageNo] – "#page=386".

Link behavior in web browsers

A web browser usually displays a hyperlink in some distinguishing way, e.g. in a different color, font or style. 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 motif to indicate a link. In most graphical web browsers, links are displayed in underlined blue text when they have not 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) the browser will display the target of the link. 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" or "id":

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

Example: To embed a link into a Page, blogpost, or comment, it may take this form:

<a href="http://example.com/">Example</a>

After publishing, the complex link string is reduced to the following for visualization in typical Web browsers:

Example

This contributes to a clean, easy to read text or document.

When the cursor hovers over a link, depending on the browser and/or 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, and many other web browsers all show the URL. In addition, the URL is commonly shown in the status bar.

Normally, a link will open 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 will be loaded. If no window exists with that name, a new window will be 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. In order to prevent accidental reuse of a window, the special window names "_blank" and "_new" are usually available, and will 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 of the hyperlink

Douglas Engelbart and his team at SRI, 1969

The term "hyperlink" was coined in 1965 (or possibly 1964) by Ted 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 (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 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) 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.

A database program HyperCard was released in 1987 for the Apple Macintosh that allowed hyperlinking between various pages within a document. The first widely used open protocol that included hyperlinks from any Internet site to any other Internet site was the Gopher protocol 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

Main article: Copyright aspects of hyperlinking and framing

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.

In the U.S., if someone has agreed (by terms of use of a website or by contract) not to link to a website without the website owner's permission, then it is illegal for that person to link to it, but that does not bar an independent person from so linking without the owner's permission. If the person who may not link is not completely barred from disclosing the destination of the would-be link, then this person may describe the destination and someone else can do the linking.

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 jurisdictions it is or has been held that hyperlinks are not merely references 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 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.

In 2000 British Telecom sued Prodigy, claiming that Prodigy infringed its patent (U.S. patent 4,873,662) on web hyperlinks. After litigation, a court found for Prodigy, ruling that British Telecom's patent did not cover web hyperlinks.

In United States jurisprudence, there is a distinction between the mere act of linking to someone else's website, and linking to content that is illegal (i.e. gambling illegal in the US) or infringing (i.e. 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. For a summary of the current status of US copyright law as to hyperlinking, see discussion regarding the Arriba Soft case.

See also

• Dead link
• Internal link
• Methods of website linking
• Object hyperlinking
• PageRank
• Xenu's Link Sleuth

Further reading

• Weinreich, Harald (2001). "The look of the link – concepts for the user interface of extended hyperlinks": 19. doi:10.1145/504216.504225. ISBN 9781581134209. Retrieved 2010-09-04. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)