The ringer equivalence number (REN) relates to the incoming-ring performance of the particular item. A "standard" analog telephone usually has a REN of 1.
To determine the appropriate number of items that can be connected to an exchange line, the "total REN" is obtained by summing the REN value of each of the items connected to the line. The "total REN" number should not exceed "4".
As an example; 2 telephones, each with a REN of 1 plus a tone ringer with a REN of 1.9 (giving a REN total of 3.9) is the maximum number of items that should be connected to that exchange line.