hyperlink

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In computing, a hyperlink is a reference in a document to an external piece of information. The most common usage is in the Internet to browse through web pages: some text in the current document is highlighted so that when clicked, the browser automatically displays another page or changes the current page to show the referenced content. The highlighted element is known as a hyperlink (or link for short) and makes a logical connection between two places in the same or different documents. Hyperlinks are the basic building block of hypertexts. For example, some key words in a wiki such as Wikipedia are highlighted, and provide links to explanations of that words at other pages in the same wiki.

In directed links the area from which the hyperlink can be activated is called its anchor (or source anchor); its target (or destination anchor) is what the link points to, which may be another location within the same page or document, another page or document, or a specific location within another page or document. Hyperlinks can also be bidirectional; in that case both linked documents will have an anchor each, leading to the other document respectively.

Links are widely used for reference within a hypertext document in order to help the end user find the reference easily without scrolling down the page or type a URL (web address) into the location bar directly.

To insert a hyperlink to another place is often simply called to "link". Hypertext (meaning "more than just text") is a form of text typically published on websites that provides a richer functionality than simple text documents by allowing the reader to learn about topics within the article by clicking on key words. Typically the link anchor will be descriptive of the target's content, for example Wikipedia home page, but badly designed or malicious sites may use obscure links or obfuscated links which make it hard to work out where the link will take you.

Types of links

Embedded link

Example: The first word of this sentence: ("Example") is a navigation link embedded in a text object – if the word is clicked, the browser will navigate to a different page.

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. Inline links may display specific parts of the content (e.g. thumbnail, low resolution preview, cropped sections, magnified sections, description text, etc.) and access other parts or the full content when needed, as is the case with print publishing software. 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 an invisible area of the screen that covers a text label or graphical images. A technical description of a hot area is a list of coordinates relating to a specific area on a screen created in order to hyperlink areas of the image to various destinations, disable linking via negative space around irregular shapes, or enable linking via invisible areas. 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

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, wikis, e-mails, Text editors, PDF documents, 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"^[1]) 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.

Legal issues

While hyperlinking among pages of Internet content has long been considered an intrinsic feature of the Internet, some websites have claimed that linking to them is not allowed without permission.

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.^[2]

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.^[3]

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.^[4] 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.^[5].^[6][7] Linking to illegal or infringing content can be sufficiently problematic to give rise to legal liability.^{[8][9][10][11][12]} For a summary of the current status of US copyright law as to hyperlinking, see this discussion.

See also

Look up hyperlink in Wiktionary, the free dictionary.

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

References

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?