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  1. [ References  ](/en-US/docs/Web)
  2. [ HTTP  ](/en-US/docs/Web/HTTP)
  3. [ HTTP headers  ](/en-US/docs/Web/HTTP/Headers)
  4. [ If-Unmodified-Since  ](/en-US/docs/Web/HTTP/Headers/If-Unmodified-Since)

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##  In this article

  * Syntax 
  * Directives 
  * Examples 
  * Specifications 
  * Browser compatibility 
  * See also 

  1. [ HTTP ](/en-US/docs/Web/HTTP)
  2. Guides 
  3. Resources and URIs 
    1. [ Identifying resources on the Web ](/en-US/docs/Web/HTTP/Basics_of_HTTP/Identifying_resources_on_the_Web)
    2. [ Data URLs ](/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URLs)
    3. [ Introduction to MIME types ](/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_Types)
    4. [ Common MIME types ](/en-US/docs/Web/HTTP/Basics_of_HTTP/MIME_types/Common_types)
    5. [ Choosing between www and non-www URLs ](/en-US/docs/Web/HTTP/Basics_of_HTTP/Choosing_between_www_and_non-www_URLs)
  4. HTTP guide 
    1. [ Basics of HTTP ](/en-US/docs/Web/HTTP/Basics_of_HTTP)
    2. [ Overview of HTTP ](/en-US/docs/Web/HTTP/Overview)
    3. [ Evolution of HTTP ](/en-US/docs/Web/HTTP/Basics_of_HTTP/Evolution_of_HTTP)
    4. [ HTTP Messages ](/en-US/docs/Web/HTTP/Messages)
    5. [ A typical HTTP session ](/en-US/docs/Web/HTTP/Session)
    6. [ Connection management in HTTP/1.x ](/en-US/docs/Web/HTTP/Connection_m