SGML is an international standard for the description of marked-up electronic text. More exactly, SGML is a metalanguage, that is, a means of formally describing a language, in this case, a markup language. Before going any further we should define these terms.
Historically, the word markup has been used to describe annotation or other marks within a text intended to instruct a compositor or typist how a particular passage should be printed or laid out. Examples include wavy underlining to indicate boldface, special symbols for passages to be omitted or printed in a particular font and so forth. As the formatting and printing of texts was automated, the term was extended to cover all sorts of special markup codes inserted into electronic texts to govern formatting, printing, or other processing.
Generalizing from that sense, we define markup, or (synonymously) encoding, as any means of making explicit an interpretation of a text. At a banal level, all printed texts are encoded in this sense: punctuation marks, use of capitalization, disposition of letters around the page, even the spaces between words, might be regarded as a kind of markup, the function of which is to help the human reader determine where one word ends and another begins, or how to identify gross structural features such as headings or simple syntactic units such as dependent clauses or sentences. Encoding a text for computer processing is in principle, like transcribing a manuscript from scriptio continua, a process of making explicit what is conjectural or implicit, a process of directing the user as to how the content of the text should be interpreted.
By markup language we mean a set of markup conventions used together for encoding texts. A markup language must specify what markup is allowed, what markup is required, how markup is to be distinguished from text, and what the markup means. SGML provides the means for doing the first three; documentation such as these Guidelines is required for the last.
The present chapter attempts to give an informal introduction---much less formal than the standard itself---to those parts of SGML of which a proper understanding is necessary to make best use of these Guidelines.