A trap and trace device is an electronic device used to record and trace all communication signals from a telecommunication system. An analogous feature available (usually at an additional charge) for use by the general public would be Caller ID.

Title 18 of the United States Code defines a trap and trace device as:

The term ?trap and trace device? means a device or process which captures the incoming electronic or other impulses which identify the originating number or other dialing, routing, addressing, and signaling information reasonably likely to identify the source of a wire or electronic communication, provided, however, that such information shall not include the contents of any communication

A trap and trace device is similar to a pen register. 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.

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