My friend is a marketing person specializing in direct mail. Recently, I got a mail piece from a pool company that had a letter inside. The part that stumped me was that the envelope didn’t have a window, and my address was on the envelope and on the letter inside. So I asked my friend, how do they know to marry up the envelope with the letter? He said this is called a two way match. In a nutshell, a machine puts an identifying barcode or sequence number on the letter or envelope. As a result, when the machine is inserting the letters into the envelopes, it reads this symbol and knows which goes with which.