A pen register is similar to a trap and trace device. A trap and trace device would show what numbers had called a specific telephone, i.e., all "incoming" phone numbers. A pen register rather would show what numbers a phone had called, i.e. all "outgoing" phone numbers. The two terms are often used in concert, especially in the context of Internet communications. They are often jointly referred to as "Pen Register or Trap and Trace devices" to reflect the fact that the same program will probably do both functions in the modern era, and the distinction is not that important. The term "pen register" is often used to describe both pen registers and trap and trace devices.