The Free On-line Dictionary of Computing (30 December 2018):

soundex

    An algorithm for encoding a word so that
   similar sounding words encode the same.  The first letter is
   copied unchanged then subsequent letters are encoded as
   follows:

   	bfpv	 -> "1"
   	cgjkqsxz -> "2"
   	dt	  	 -> "3"
   	l	  	 -> "4"
   	mn	  	 -> "5"
   	r	  	 -> "6"

   Other characters are ignored and repeated characters are
   encoded as though they were a single character.  Encoding
   stops when the resulting string is four characters long,
   adding trailing "0"s if it is shorter.  For example, "SMITH"
   or "SMYTHE" would both be encoded as "S530".

   (1995-01-05)