SGML is the ISO ‘Standard Generalized Markup Language.’ It defines a powerful language for describing and documenting hierarchically structured documents of arbitrary complexity with simple character stream files. It does not specify a particular set of content object types or ‘tags,’ but rather provides a way for declaring which tags are to be used, along with their permissible relationships. For documents of fixed form such as dictionaries and reference works this can be a great help in establishing a consistent structure. Even in the case of more loosely structured material such as literary texts, the existence of a precise description of the document structure can be of use in analysis. For these texts SGML may be more an aid to the scholar than the author. SGML defines a document in terms of its OHCO structure: it does not directly specify how to format or process a document, but describes a hierarchical document structure with mnemonic names for the content objects of the data. Thus, it does not prejudice whether a document is to be treated as a database, a word-processing file, or something completely different. It is important to note, however, that this independence does not prevent an SGML-using application from displaying data in any way the user desires. Many products provide tools for assigning an appearance to each content object type, and a “WYSIWYG” display for writing and editing.