## 29.4 Document Type Definitions

- DTD - set of rules for correctly formed documents of a particular type
- E.g., consider the `items` document:
  - `items` denotes a sequence of `item` elements
  - `item` contains a `product` and a `quantity`
  - `product` contains a `description` and a `price`
  - Each of these elements contains descriptive text
- DTD formalizes this description

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