Emacs is an advanced and extensible editor. An Emacs major mode customizes Emacs for editing particular types of text documents. PSGML is a major mode for SGML (a markup language) documents. PSGML provides several functionalities for editing SGML documents: indentation according to element nesting depth and identification of structural errors (but it is not a validating SGML parser); menus and commands for inserting tags with only the contextually valid tags; attribute values can be edited in a separate window with information about types and defaults; structure based editing includes movement and killing; and also several commands for folding editing.