Hypertext References: Link Types

Link Relationship Values in HTML

Source

In the section entitled Link Relationship Values of the 15 March 1993 version of Hypertext Markup Language (HTML) Tim Berners-Lee and Daniel Connolly listed suggested values for HTML's REL attribute. As I have not seen evidence of these proposed values anywhere else I am providing a transcription of that list here. Other values were proposed in the now expired HTML 3.0 draft.

You can use the table of contents to jump directly to parts of the list.

Note that the definition of the REL and REV in version 2.0 of the HTML standard make it clear that the relationships described apply to links and not documents. The definition of hyperlink on which they are based states that the relationship is between anchors (element A).

Status

They wrote that the following list is not part of the [proposed] standard but is intended to illustrate the use of link relationships and to provide a framework for further development. They further wrote that

Additions to this list will be controlled by the HTML registration authority. Experimental values may be used on the condition that they begin with "X-". ...These relationships relate whole documents (objects), rather than particular anchors within them. If the relationship value is used with a link between anchors rather than whole documents, the semantics are considered to apply to the documents.

In the explanations which follows, A is the source document of the link and B is the destination document specified by the HREF attribute.

A relationship marked "Acyclic" has the property that no sequence of link with that relationship may be followed from any document back to itself. These types may therefore be used to define trees.

The list

Relationships between documents

These relationships are between the documents themselves rather than the subjects of the documents.

UseIndex

B is a related index for search by a user reading this document who asks for an index search function.
A document may have any number of index links, causing several indexes top [sic] be searched in a client-defined manner.
B must support SEARCH operations under its access protocol

UseGlossary

B is an index which should be used to resolve glossary queries in the document. (Typically, a double-click on a word which is not within an anchor).
A document may have any number of glossary links.

Annotation

The information in B is additional to and subsidiary to that in A.
Annotation is used by one person to write the equivalent of `margin notes' or other criticism on another's document, for example.
Example: The relationship between a newsgroup and its articles.
Acyclic.

Reply

Similar to Annotation, but there is no suggestion that B is subsidiary to A: A and B are on equal footings.
Example: The relationship between a mail message and its reply, a news article and its reply.
Acyclic.

Embed

If this link is followed, the node at the end of it is embedded into the display of the source document.
Acyclic.

Precedes

In an ordered structure defined by the author, A precedes B, B is followed by A.
Acyclic.
Any document may only have one link of this type, and/or one reverse link of this relationship.
Note: May be used to control navigational aids, generate printed material, etc. In conjunction with Includes, may be used to define a tree such as a printed book made of hypertext document. The document can have only one such tree.

Present

Whenever A is presented, B must also be presented. This implies that whenever A is retrieved, B must also be retrieved.

Search

When the link is followed, the node B should be searched rather than presented. That is, where the client software allows it, the user should immediately be presented with a search panel and prompted for text. The search is then performed without an intermediate retrieval or presentation of the node B[.]

Supersedes

B is a previous version of A.
Acyclic.

History

B is a list of versions of A.
A link reverse link must exist from B to A and to all other known versions of A.

Relationships about subjects of documents

Includes

A includes B, B is part of A. For example, a person described by document A is a part of the group described by document B.
Acyclic.

Made

Person (etc[.]) described by node A is author of, or is responsible for B[.]
This information can be used for protection, and informing authors of interest, for sending mail to authors, etc.

Interested

Person (etc[.]) described by A is interested in node B[.]
This information can be used for informing readers of changes.