title: self completing code
body: <p><a href="https://electronics.stackexchange.com/questions/20537/how-to-identify-self-complementing-code">https://electronics.stackexchange.com/questions/20537/how-to-identify-self-complementing-code</a></p>
  
  <p>I have read that properly , </p>
  
  <p>but is it necessary that , the number need to be subtracted from 9  ?</p>
  
  <p>because  in the 8421 system 
   12= 1100</p>
  
  <p>and if we subtract 15-12 , then  the answer is 3 , which is 0011 . </p>
  
  <p>so it is also self complementing...</p>
  
  <p>I don't understand that , the given number need to be subtracted from which number , is that number neccessary to be 9  ???</p>