I'd like to learn something from the design of HTTP protocol - why is there, for example, "Not found", following the "404" code? Wouldn't the number itself be sufficient?
closed as not constructive by Kenny Rasschaert, Bryan, mdpc, Ward♦, Zoredache Jan 14 '13 at 21:34
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6.1.1 Status Code and Reason Phrase
The Status-Code element is a 3-digit integer result code of the attempt to understand and satisfy the request. These codes are fully defined in section 10. The Reason-Phrase is intended to give a short textual description of the Status-Code. The Status-Code is intended for use by automata and the Reason-Phrase is intended for the human user. The client is not required to examine or display the Reason- Phrase.
Basically it allows the server to be more detailed about the response in case the application decides to display it. I've never seen it used in practice, but the protocol allows it.