Simple Type Definitions
 
  - Treats attributes and sub-elements the same.
- A frequently-expressed requirement for XML.
- We need an inventory of simple types for strings.
- Built-in simple types:
   
    - boolean, decimal, uriReference, hexBinary, dateTime, duration, . . .
-  QName, NOTATION, . . .
-  integer, NCName, ID, IDREFS, . . .