A pen register is a device or process that traces outgoing signals from a specific phone or computer to their destination; it is often used by law enforcement as the advanced counterpart of an outgoing call log. A pen register produces a list of the phone numbers or IP addresses contacted, but does not include substantive information transmitted by the signals.
Compare: trap and trace devices
In Smith v. Maryland, the United States Supreme Court held that installation and use of a pen register by a telephone company does not constitute a “search” within the context of the Fourth Amendment. The Court distinguished pen registers from listening devices because pen registers “do not acquire the contents of communications.” Therefore, the telephone numbers obtained through the pen register were properly introduced as evidence in this case even though the pen register was installed without a search warrant.
- 18 U.S. Code Chapter 119 - Wire and electronic Communications Interception and Interception of Oral Communications
- 18 U.S. Code Chapter 206 - Pen Registers and Trap and Trace Devices
- 18 U.S. Code § 3121 - General prohibition, exceptions, and limitations on the use of pen registers and trap and trace devices.
- 18 U.S. Code § 3122 - Outlines the application for an order for a pen register or a trap and trace device.
- 18 U.S. Code § 3123 - Provides the issuance of an order for a pen register or a trap and trace device.
- 18 U.S. Code § 3124 - Regarding assistance in installation and use of a pen register or a trap and trace device.
- 18 U.S. Code § 3125 - Emergency pen register and trap and trace device installation.
- 18 U.S. Code § 3126 - Reports concerning pen registers and trap and trace devices.
[Last updated in November of 2023 by the Wex Definitions Team]