SGML allows documents to include markup that describes the structure of the document, and provides a method of specifying the desired structure for a document.

For example, the Complete Works of Shakespeare might be assigned the logical structure shown below.

This logical structure might then be described in an SGML DTD as:

<!-- A work has a header identifying some known properties of the work and body -->

    <!ELEMENT	WORK  	   - - (HEADER, BODY)>
    

<!--The header collects together some useful information about a work. Id is a unique numeric identifier for the work. Author identifies the probable author of the work. -->

    <!ELEMENT	HEADER	   - - (ID, AUTHOR)>
    <!ELEMENT	ID         - - (#PCDATA)>
    <!ELEMENT	AUTHOR     - - (#PCDATA)>
    

<!-- The body of each work has a title, and a possible scenario. It then has acts followed by an optional list of characters. Songs, drama and asides may occur anywhere within the body -->

    <!ELEMENT	BODY       - - (TITLE, SCENARIO?, ACT*,CHARACTERS?) +(ASIDE)>
    <!ELEMENT	TITLE      - - (#PCDATA)>
    

<!--The character list describes (sets of related) characters in the play Characters may be related either because they form a natural collection or because of a natural relationship between them. -->

    <!ELEMENT	CHARACTERS - - (RELATED | CHARACTER)*>
    <!ELEMENT	RELATED    - - (COLLECTION?, GROUP?, BECAUSE?)>
    <!ELEMENT	COLLECTION - - (#PCDATA)>
    <!ELEMENT	GROUP      - - (CHARACTER+)>
    <!ELEMENT	BECAUSE    - - (#PCDATA)>
    

<!--Each character has zero or more names (i.e. titles), a possible role within the play, and zero or more short handles used to identify this character as a speaker -->

    <!ELEMENT	CHARACTER  - - (NAME*, ROLE?, SHORT*)>
    <!ELEMENT	NAME       - - (#PCDATA)>
    <!ELEMENT	ROLE       - - (#PCDATA)>
    <!ELEMENT	SHORT      - - (#PCDATA)>
    

<!--Each play has a scenario (where it was set) and a collection of acts Each act contains one or more scenes. Acts and scenes have numbers or text which distinguish them within a play. Scenes have an associated set (instructions for the stage setting for the scene) -->

    <!ELEMENT	SCENARIO   - - (#PCDATA)>
    <!ELEMENT	ACT        - - (ACTNO,SCENE+) +(SONG, DRAMA)>
    <!ELEMENT	ACTNO      - - (#PCDATA) -(SONG, DRAMA)>
    <!ELEMENT	SCENE      - - (SCENENO, SET?, SPEECH*)>
    <!ELEMENT	SCENENO    - - (#PCDATA) -(SONG, DRAMA)>
    <!ELEMENT	SET        - - (#PCDATA) -(SONG, DRAMA)>
    

<!-- Each speech has zero or more speakers, and either involves singing or saying something. An aside is any parenthetical instructions, comment etc. Song is anything to be sung. Drama is anything which is to be presented by the actors. -->

    <!ELEMENT	SPEECH     - - (SPEAKER*, (SAID | SONG))>
    <!ELEMENT	SPEAKER    - - (#PCDATA)>
    <!ELEMENT	SAID       - - (#PCDATA)>
    <!ELEMENT	ASIDE      - - (#PCDATA)>
    <!ELEMENT	SONG       - - (#PCDATA)>
    <!ELEMENT	DRAMA      - - (#PCDATA)>