Hyperlink

In computing, a hyperlink (or link) is a reference to a document that the reader can directly follow, or that is followed automatically. The reference points to a whole document or to a specific element within a document. Hypertext is text with hyperlinks. Such text is usually viewed with a computer. A software system for viewing and creating hypertext is a hypertext system. To hyperlink (or simply to link) is to create a hyperlink. A user following hyperlinks is said to navigate or browse the hypertext.

A hyperlink has an anchor, which is a location within a document from which the hyperlink can be followed; that document is known as its source document. The target of a hyperlink is the document, or location within a document, that the hyperlink leads to. The user can follow the link when its anchor is shown by activating it in some way (often, by touching it or clicking on it with a pointing device). Following has the effect of displaying its target, often with its context.

In some hypertext, hyperlinks can be bidirectional: they can be followed in two directions, so both points act as anchors and as targets. More complex arrangements exist, such as many-to-many links.

The most common example of hypertext today is the World Wide Web: webpages contain hyperlinks to webpages. 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 that predate the computer, such as tables of contents, footnotes, bibliographies, indexes and glossaries.

The effect of following a hyperlink may vary with the hypertext system and sometimes on the link itself; for instance, on the World Wide Web, most hyperlinks cause the target document to replaces 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. A third option exists when an automatic software program traverses the hypertext following each hyperlink and gathering all the retrieved documents. That program is said to be spidering or crawling the hypertext.

Types of links

External links

The word example in this sentence is a hyperlink that points to a different (external document) – if the word is clicked, the browser will navigate to a different page. Such links are called navigation links or external links.

Inline link

An 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.

A 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. 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.

Hot area

A hot area (image map in HTML) is a hyperlink anchor formed by a designated, often irregular part of an image or of other content. 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 irregularly shaped 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.

Random accessed

Random-accessed linking data are links retrieved from a database or variable containers in a program when the retrieval function is from user interaction (e.g. dynamic menu from an address book) or non-interactive (e.g. random, calculated) process.

Hardware accessed

A hardware-accessed link is a link that activates directly via an input device (e.g. keyboard, microphone, remote control) without the need or use of a graphical user interface.

Hyperlinks in various technologies

Hyperlinks in HTML

Tim Berners-Lee saw the possibility of using hyperlinks to link any unit of information to any other unit of 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 language 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">" which marks the start of an anchor, followed by the highlighted text and the "</a>" symbol indicating the end of the source anchor. The <a> element can also be used to indicate the target of a link.

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 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.

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. 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 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 — this is a fragment identifier.

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 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 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 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 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:

• 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). 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. Windows can be named upon creation, and that identifier can be used to refer to it later in the browsing session. If no current window exists with that name, a new window will be created using the ID.

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

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. 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.

A database program HyperCard was released in 1987 for the Apple Macintosh that allowed hyperlinking between various types of pages within a document.

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 to from 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 another entry point designated by the site owner, but to content elsewhere, allowing the user to bypass the site's own designated flow;
• 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, for example, Karin Spaink was initially convicted of copyright infringement for linking, although this ruling was overturned in 2003. The courts that advocate 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. 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 or infringing. Several courts have found that merely linking to someone else's website is not copyright or trademark infringement, regardless of how much that 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 this discussion.

See also

• Dead link
• HTML element
• Internal link
• Object hyperlinking
• Overlinking
• Underlinking
• Xenu's Link Sleuth — checks Web sites for broken hyperlinks]]
• Link Diving

External links

• Links in HTML 4 (w3.org)
• Uniform Resource Locators (ietf.org)
• Relative Uniform Resource Locators (ietf.org)
• A Brief History of the Hyperlink
• Links & Law — Overview of legal issues and court rulings involving linking
• The Expert's Guide to Using Hyperlinks
• Hyperlink Project
• Cybertelecom:: Legal to Link?