title: (K&R) At least the first 31 characters of an internal name are significant?
body: <p>When taken literally, it makes sense, but what exactly does it mean to be a significant character of a variable name?</p>
  
  <p>I'm a beginning learner of C using K&amp;R. Here's a direct quote from the book:</p>
  
  <blockquote>
    <p>"At least the first 31 characters of an internal name are significant. For function names and external variables, the number may be less than 31, because external names may be used by assemblers and loaders over which the language has no control. For external names, the standard guarantees only for 6 characters and a single case."</p>
  </blockquote>
  
  <p>By the way, what does it mean by "single case"?</p>