m HypermedialTime-based Document Struc:turing Language he computer and telecom- ment contains data consisting of exam- munications industries have ples of markup (for example, an made enormous progress in SGML document about SGML). communications technology stan- The proposed standard "HyTime" dardization in recent years. One Hypermedia/Time-based Document effect of good communications tech- Structuring Language (ISO/IEC nology is that people can concentrate Draft International Standard (DIS) on the information being communi- 10744), built on the Standard Gener- cated. More and more people are alized Markup Language (SGML; realizing, however, that being able ISO/IEC International Standards to send and receive files containing (IS) 8879-1986), is designed to make information is not enough. all this possible. By using SGML/ It is desirable that all digital HyTime, all kinds of documents can documents explicitly indicate in a package their information content standard way what kind of notation using standard "markup." This is used in them. When an electronic markup provides information about document is created, its author the structure and notation(s) of the should be able to incorporate active document in a way that is under- references to other on-line documents standable or interpretable by any ("hyperlinks"l), regardless of the application that has been provided heterogeneity of their notations. In other words, there with an appropriate data importation facility. The should be a standard way to make "information about "structured" character of such documents will also information" interoperable. Such a standard should, make them amenable to nonsequential browsing, among other things: querying, access and version control, and maintenance over very long time spans. • provide a standard way to express the fact that any number of data objects in any number of different nota- The WOrld Of Structured tions are related in some way or for some reason. 2 Documents • provide a standard way to express the way in which The majority of the information stored by human any number of data objects in any number of dif- civilization is for perception by human beings? Most of ferent notations are intended to be rendered for human that information is fully formatted for immediate and perception in space, in time, or in both space and time? direct perception, and stored on printed pages. Some One method of incorporating information about the of it is fully formatted, but still requires the use of some information contained in a document is to use "markup ''4 mechanism to render it perceivable. In this category consisting mainly of "start tags" and "end tags" that are such items as magnetically recorded plain ASCII respectively precede and follow each logical portion of text files, PostScript files, digital video and sound a document. Tags must be specially punctuated so that recordings, etc. The rest of the information is stored the markup can be identified as markup when the docu- unformatted, but in such a way that is convenient to ment is parsed, and it can be processed separately from format, perform, or otherwise render for direct percep- the data which it surrounds. Obviously, the rules for speci- tion by human beings; databases and structured fying the punctuation that distinguishes markup from documents fall into the latter category. data must be powerful enough to ensure that the markup Structured documents are so named because the hierar- and the data cannot be confused even when a given docu- chical and sequential structure of the various kinds of Steven R. New¢omb, Neill A. Kipp, and Victoria 1'. New¢omb COMMUNICATIONS OF THE ACM/November 1991/Voi.34, No.ll 67 • iv-a information they contain is made ex- document is: "Show me the authors industry in its own right, very dis- plicit by identifying tags. Each tag and titles of the documents whose tinct from the hardware and sys- associates a "generic identifier ''L chapter titles contain both 'napo- tems industries, and in need of a the name of the kind of thing being leon' and either 'frosting' or way to protect its product invest- tagged (e.g., "subsection")--with the 'icing.'" ments from losing value as a result data surrounded by a start tag and Communities of interest can of changes in hardware technology. end tag of the same generic identifier. agree on a set of generic tags for One remarkable feature of Generic tags bear a superficial their documents, and they can HyTime is its ability to accommo- resemblance to the embedded for- thereby immunize their documents date and support musical informa- matting codes used by virtually all against the obsolescence of their tion in a fashion that will allow such text-processing applications, in that formatting systems, and at the same information to be completely inte- they occur mixed in with data, they time maximize the availability of grated with other kinds of informa- can affect the way the data are for- the information they have collec- tion: to share the same scheduling matted, and they do not appear in tively created. Moreover, the infor- facilities, to be hyperlinkable, etc. the formatted document. An exam- mation in generically tagged docu- Without a deliberate design effort ple of an embedded formatting ments remains available for uses toward a holistic hypermedia struc- code is one which causes the text on totally unforeseen when the docu- turing language, the representa- the following line to be centered, ment was created. A community of tional needs of whole areas of e.g., .oe in an nroff document. We interest can further agree about the human endeavor, such as music, might use such an embedded in- contexts in which each generis may could be unintentionally prevented struction to center the title of an appear, e.g., "no chapter titles may from participating fully in our in- article. However, generic tags and appear within any paragraph." The creasingly information-driven civi- formatting codes are philosophi- agreement can further elaborate lization. It seems unlikely that a cally in different universes. The each generis with a list of allowable single-industry-driven de facto stan- generic tags in structured docu- attributes and attribute values-- dard would be capable of incorpo- ments do not have the effect of as- information about a paragraph, for rating facilities that would meet the sociating some particular format- example, which is not part of the content of that paragraph. IThe word hyperlink is deliberately used in this ting instruction with some data. article instead of the more familiar bypertext That is, in the case of data tagged as jargon link because the word link has a special Why should there be a structuring a title, '.no particular formatting in- meaning in SGML. In SGML, a link is a con- language for multimedia and struct used to associate special processing in- struction is implied; the tags merely hypermedia documents? structions with portions of a document; it has identify the title as a title. How a People share common languages nothing to do with hypertext. title is to be formatted is entirely a because they need to communicate 2This is accomplished by the document loca- matter to be decided when the doc- with one another. A common struc- tion address module and the hyperlink mod- ule of HyTime. ument is formatted. turing language for hypermedia A formatting application for a documents is needed in order to SThis is accomplished by HyTime's finite co- ordinate space (FCS) module, including two generically tagged document can be permit human communications in FCS submodules, the event projection mod- driven by a "style sheet"--a table this relatively new combination of ule and the object modification module. that associates a set of formatting media, given that the computing 4Markup is so named because of its resem- instructions with each generic iden- environments we create for our- blance to the markings that editors make on drafts of paper documents. tifier, i[n the case of print docu- selves are now and will probably ments, title, for example, may be always be heterogeneous. 5Comparatively little information is stored in a form intended solely for machine process- associated with "boldface, 14-point, HyTime's design reflects the ing, e.g., as a succession of predicate calculus centered text" and paragraph with view that all multimedia and hyper- statements, or as an image of a neural net- "Roman, 10-point, ragged right." If text technologies, all graphics tech- work. a document is tagged generically, it nologies, all audio technologies, 6The phrase generic identifier is SGML jargon. It emphasizes the idea that the identifier is is generally unnecessary for anyone etc., regardless of whether or not used to say what kind of thing is being identi- to edit it in order to reformat it ac- they are proprietary, should be al- fied. It would be just as valid (but not in keep- cording to a different style. All that lowed to compete with one another ing with tradition) to say structural identifier, thus emphasizing the importance of context is necessary is to use a different in a market environment which is of the kind of object being identified. style sheet with the formatting ap- technically able to accommodate 7Nroffis a venerable text-processing program, plication, s any combination of them in any in- usually supplied with Unix-based computer Generic tagging offers another formation product. It also reflects a systems. even more significant benefit: col- conviction that "software" (in the 8The SGML "link" feature (which, as noted lections of generically tagged docu- broadest sense that includes such previously, has nothing whatsoever to do with ments can be queried like data- things as movies, recordings, hyperlinking) does provide an explicit way to associate formatting instructions with the in- bases.