Sonia Sotomayor just became the third woman to move from the appellate bench to the U.S. Supreme Court. The first woman on the nation's highest court has gone in the opposite direction.

Though she retired in 2006 to look after her ailing husband, Justice Sandra Day O'Connor is still out there judging. Unbeknown even to some of her former colleagues on the Supreme Court, the 79-year-old jurist has been visiting federal appellate courts across the country, filling in as a substitute judge when vacations or vacancies leave their three-member panels understaffed.

"It's nice to keep your hand in a bit," she said in an interview in the chambers she still keeps at the Supreme Court.

As a substitute judge, Justice O'Connor has heard nearly 80 cases and written more than a dozen opinions. In her 24-year Supreme Court tenure, she often provided the pivotal vote on such issues as abortion, affirmative action and religious freedom. Nowadays, she decides such matters as whether a drug dealer could escape punishment because a search warrant listed one household trash can instead of two.

O'CONNOR ON U.S. V. LABOY-TORRES

STEPHEN VOSS FOR THE WALL STREET JOURNAL

Sitting as a substitute judge on the Third U.S. Circuit Court of Appeals in Philadelphia, Justice O'Connor delivered the opinion in U.S. v. Laboy-Torres. At issue: Are Puerto Rican courts foreign or domestic? See the key documents:

Marco Laboy-Torres's appeal brief

Fritz Ulrich, a public defender from Harrisburg, Pa., came