The swapper is placed between the RJ"2"1 phone jack, and the DSL modem. It will change, or swap pins 4 & 5, to pins 2 & 3. This allows a wall jack wired for DSL on pins 4 & 5 to connect to the ADSL modem that expects the DSL on pins 3 & 4. If a customer has two phone lines, his DSL modem line may be on pins 4 & 5 and require a swapper. A swapper may be needed even if a user does not have a second line. Sometimes, when a splitter is installed (by the service provider), the installer may place regular voice (POTs) on pins 3 & 4, and the DSL data on pins 4&5. A swapper will be needed in these cases.