Pen register

A pen register, or dialed number recorder (DNR), is an electronic device that records all numbers called from a particular telephone line. The term has come to include any device or program that performs similar functions to an original pen register, including programs monitoring Internet communications.

The United States statutes governing pen registers are codified under 18 U.S.C., Chapter 206.

The term pen register originally referred to a device for recording telegraph signals on a strip of paper. Samuel F. B. Morse's 1840 telegraph patent described such a register as consisting of a lever holding an armature on one end, opposite an electromagnet, with a fountain pen, pencil or other marking instrument on the other end, and a clockwork mechanism to advance a paper recording tape under the marker.

The term telegraph register came to be a generic term for such a recording device in the later 19th century. Where the record was made in ink with a pen, the term pen register emerged. By the end of the 19th century, pen registers were widely used to record pulsed electrical signals in many contexts. For example, one fire-alarm system used a "double pen-register", and another used a "single or multiple pen register".

As pulse dialing came into use for telephone exchanges, pen registers had obvious applications as diagnostic instruments for recording sequences of telephone dial pulses. With the passage of time, any instrument that could be used for this purpose came to be defined as a pen register. Title 18 of the United States Code defines a pen register as:

a device or process which records or decodes dialing, routing, addressing, or signaling information transmitted by an instrument or facility from which a wire or electronic communication is transmitted, provided, however, that such information shall not include the contents of any communication, but such term does not include any device or process used by a provider or customer of a wire or electronic communication service for billing, or recording as an incident to billing, for communications services provided by such provider or any device or process used by a provider or customer of a wire communication service for cost accounting or other like purposes in the ordinary course of its business

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