A hyperlink has an anchor, which is the location within a certain type of a document from which the hyperlink can be followed only from the homepage; the document containing a hyperlink is known as its source code 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.[wikipedia]