title: Likeliest way to compute validation using MOD 7 check on the last number
body: <p>I am attempting to perform number validation for a proprietary ID implementation. I was anticipating a spec document for the algorithm in detail, but this is all that they sent:</p>
  
  <pre><code>The IDs are 9 digits.
  The check digit is a MOD 7 check on the last number.
  </code></pre>
  
  <p>I think I am to assume that "MOD 7" means to apply modulus 7 to something--I suspect that "the last number" means more than just applying mod 7 to the last digit, otherwise every valid ID would end in 0 or 7.</p>
  
  <p>Meanwhile someone in a separate conversation said that it was actually "a CRC MOD 7 check on the last number" (throwing in "CRC"), but I <em>think</em> that this was a misspoken detail and that CRC is not used at all, but what do I know?</p>
  
  <p>I am having difficulty finding information on some standard way of reading this specification and interpreting this for some "standard algorithm". Most of the samples I've found consist of many different varieties of algorithms, such as weighted or unweighted, etc.</p>
  
  <p>Does anyone know how I am most likely to interpret this, or if I am supposed to ask for more information? If I am supposed to ask for more information, what questions should I ask?</p>