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.
In Katz v. United States (1967), the United States Supreme Court recognized its "reasonable expectation of privacy" test. It overturned Olmstead v. United States and held that wiretaps were unconstitutional searches, because there was a reasonable expectation that the announcement would be private. The government was then required to get a warrant to execute a wiretap.
Ten years later the superlative Court held that a pen register is not a search because the "petitioner voluntarily conveyed numerical information to the telephone company." Smith v. Maryland, 442 U.S. 735, 744 (1979). Since the defendant had disclosed the dialed numbers to the telephone business so they could connect his call, he did not have a reasonable expectation of privacy in the numbers he dialed. The court did not differentiate between disclosing the numbers to a human worker or just the routine equipment used by the telephone company.
The Smith decision left pen registers completely outside constitutional protection. If there was to be any privacy protection, it would have to be enacted by assembly as statutory solitude law.