Home > Articles

This chapter is from the book

DTD Construction Basics

You already know that DTDs represent the original schema technique supported by XML, and are tightly integrated with the documents they describe. Therefore, to understand how DTDs are constructed, you must first understand how they relate to XML documents. In order to use a DTD with a document, you must somehow associate the DTD with the document. This association is carried out through the document type declaration, which must be placed at the beginning of an XML document just after the XML declaration:

<?xml version="1.0"?>
<!DOCTYPE talltales SYSTEM "talltales.dtd">

The second line in this code is the document type declaration for the Tall Tales document you saw earlier. The main thing to note about this code is how the Tall Tales DTD is specified.

The terminology surrounding DTDs and document type declarations is admittedly confusing, so allow me to clarify that DTD stands for document type definition and contains the actual description of a markup language. A document type declaration is a line of code in a document that identifies the DTD. So, the big distinction here is that the definition (DTD) describes your markup language, whereas the declaration associates it with a document. Got it? Let’s move on!

A DTD describes vital information about the structure of a document using markup declarations, which are lines of code that describe elements, attributes, and the relationship between them. The following types of markup declarations may be used within a DTD:

  • The elements allowed in the document
  • The attributes that may be assigned to each element
  • Entities that are allowed in the document
  • Notations that are allowed for use with external entities

Elements and attributes you know about, but the last two markup declarations relate to entirely new territory. Don’t worry because you learn more about entities and notations in the next hour, "Digging Deeper into XML Documents." For now, it’s important to understand that the markup declarations within a DTD serve a vital role in allowing documents to be validated against the DTD.

When associating a DTD with a document, there are two approaches the document type declaration can take:

  • It can directly include markup declarations in the document that form the internal DTD.
  • It can reference external markup declarations that form the external DTD.

These two approaches to declaring a DTD reveal that there are two parts to a DTD: an internal part and an external part. When you refer to the DTD for a document, you are actually referring to the internal and external DTDs taken together. The reason for breaking the DTD into two parts has to do with flexibility. The external DTD typically describes the general structure for a class of documents, whereas the internal DTD is specific to a given document. XML gives preference to the internal DTD, which means you can use it to override declarations in the external DTD.

Breaking Down a DTD

The following code shows the general syntax of a document type declaration:

<!DOCTYPE RootElem SYSTEM ExternalDTDRef [InternalDTDDecl]>

The external DTD is referenced by ExternalDTDRef, which is the URI (Uniform Resource Identifier) of a file containing the external DTD. The internal DTD corresponds to InternalDTDDecl and is declared between square brackets ([]). In addition to the internal and external DTDs, another very important piece of information is mentioned in the document type declaration: the root element. RootElem identifies the root element of the document class in the document type declaration syntax. The word SYSTEM indicates that the DTD is located in an external file. Following is an example of a document type declaration that uses both an internal and external DTD:

<!DOCTYPE talltales SYSTEM "TallTales.dtd"> [
<!ELEMENT question (#PCDATA)> ]>

This code shows how you might create a document type declaration for the Tall Tales trivia sample document. The root element of the document is talltales, which means that all documents of this type must have their content housed within the talltales element. The document type declaration references an external DTD stored in the file TallTales.dtd. Additionally, an element named question is declared as part of the internal DTD. Remember that internal markup declarations always override external declarations of the same name if such declarations exist. It isn’t always necessary to use an internal DTD if the external DTD sufficiently describes a language, which is often the case.

In the previous hour you learned about the XML declaration, which must appear at the beginning of a document and indicates what version of XML is being used. The XML declaration can also contain additional pieces of information that relate to DTDs. I’m referring to the standalone status and character encoding of a document. The standalone status of a document determines whether or not a document relies on any external information sources, such as an external DTD. You can explicitly set the standalone status of a document using the standalone document declaration, which looks like an attribute of the XML declaration:

<?xml version="1.0" standalone="no"?>

A value of yes for standalone indicates that a document is standalone and therefore doesn’t rely on external information sources. A value of no indicates that a document is not standalone and therefore may rely on external information sources. Documents that rely on an external DTD for validation can’t be considered standalone, and must have standalone set to no. For this reason, no is the default value for standalone.

Pondering Elements and Attributes

The primary components described in a DTD are elements and attributes. Elements and attributes are very important because they establish the logical structure of XML documents. You can think of an element as a logical unit of information, whereas an attribute is a characteristic of that information. This is an important distinction because there is often confusion over when to model information using an element versus using an attribute.

A useful approach to take when assessing the best way to model information is to consider the type of the information and how it will be used. Attributes provide tighter constraints on information, which can be very helpful. More specifically, attributes can be constrained against a predefined list of possible values and can also have default values. Element content is very unconstrained and is better suited for storing long strings of text and other child elements. Consider the following list of advantages that attributes offer over elements:

  • Attributes can be constrained against a predefined list of enumerated values.
  • Attributes can have default values.
  • Attributes have data types, although admittedly somewhat limited.
  • Attributes are very concise.

Attributes don’t solve every problem, however. In fact, they are limited in several key respects. Following are the major disadvantages associated with attributes:

  • Attributes can’t store long strings of text.
  • Attributes can’t contain nested information.
  • Whitespace can’t be ignored in an attribute value.

Given that attributes are simpler and more concise than elements, it’s reasonable that you should use attributes over child elements whenever possible. Fortunately, the decision to use child elements is made fairly straightforward by the limitations of attributes: if a piece of information is a long string of text, requires nested information within it, or requires whitespace to be ignored, you’ll want to place it in an element. Otherwise, an attribute is probably your best choice. Of course, regardless of how well your document data maps to attributes, it must have at least a root element.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020