title: In the C++ Boost libraries, why is there a ".ipp" extension on some header files
body: <p>In the C++ Boost libraries, why is there a ".ipp" extension on some header files?  </p>
  
  <p>It seems like they are header files included by the ".hpp" file of the same name.  </p>
  
  <p>Is this convention common outside of Boost?  </p>
  
  <p>What is the justification for having a special file type?  </p>