WASHINGTON - For years, Justice Sandra Day O'Connor's opinion set the direction for the Supreme Court on a host of issues, including important social concerns such as abortion, affirmative action and the way the nation's elections are financed.
But later this month, the retired justice herself has an opinion - in a voting registration case from her home state of Arizona - that the Supreme Court will review.
O'Connor was in the panel majority that said Arizona's requirement of proof of citizenship when registering to vote in federal elections conflicted with federal law. An en banc decision of the U.S. Court of Appeals for the 9th Circuit affirmed the panel's ruling.
O'Connor's role in the case brings attention to a little-noticed aspect of a justice's legacy: The work often continues long after leaving the high court.
Justices are appointed for life, of course. But since 1937, they have been allowed to take senior status and continue to serve on lower federal courts. The idea grew out of President Franklin D. Roosevelt's ill-fated court-packing plan.
According to a study by Minor Myers, a professor at Brooklyn Law School, 25 justices have taken senior status since then and 11 of them served on lower courts.
Some have been prolific. Myers reports that former justice Tom Clark, who retired from the court in 1967, began hearing cases in 1970 and over the next seven years sat on panels that produced nearly 400 decisions. He served on all of the nation's geographic circuits.
Clark and Willis Van Devanter, the first justice to take advantage of senior status, also presided over trials as district judges.
Justice Lewis Powell regularly heard cases on the U.S. Court of Appeals for the 4th Circuit in Richmond, Va., his home, after his retirement in 1987. But a fellow justice, Potter Stewart, did not find the life to his liking.
It was "no fun to play in the minors after a career in the major leagues," Stewart said.
In the past quarter-century, retired justices William Brennan, Thurgood Marshall and Byron White also served.
Among the current justices on senior status, John Paul Stevens decided when he left the high court in 2010 - at age 90 - that he would not sit on the lower courts. Instead, he lectures and writes, including a 2011 book "Five Chiefs," about the chief justices with whom he served.
Justice David Souter, 73, is a regular on the federal appeals court in Boston, where he served briefly before his nomination to the high court in 1990. Stephen Wasby, a University of Albany professor emeritus of political science who has studied the former justices, describes Souter in an upcoming study as "someone laboring in quiet workmanlike fashion like most court of appeals judges and as befits his low-key personality."
None of his opinions have come before the Supreme Court.
O'Connor, 82, has served on most of the nation's appeals courts since she left the high court in 2006. But of course that is only a small part of her work: She has served on commissions, been outspoken on the subject of judicial independence, launched an online civics initiative aimed at middle-schoolers and just released a book about the court's history, "Out of Order."
Wasby, who says O'Connor's lower-court work results in "moderate-to-conservative outcomes, pretty much what we would expect from her Supreme Court service," said the Arizona decision is the first of hers to be accepted for review by the justices.
She served on the panel, but not the en banc review. (As an example of the rarified world of appellate jurisprudence, both opinions were written by Judge Sandra Segal Ikuta - a former O'Connor clerk.)
According to Myers's research, retired justices do not fare particularly well at the high court.
Although he looked only at opinions written by former justices, rather than decisions in which they played a role, he found that the court vacated the only opinion it reviewed by former justice Stanley Reed. The court overturned two of the three opinions it considered written by the prolific Clark.
But former justice Charles Whittaker probably got the worst treatment from a former colleague.
The request to have a retired justice serve is made from a court to the chief justice. Whittaker, who left the high court in 1962, was prepared to serve. But Myers quotes from a book by David Atkinson that says Chief Justice Earl Warren nixed the idea of Whittaker.
Warren reportedly told another judge, "Tell him that I never could get him to make up his mind, and I'll be damned if I will let him do that to me again trying cases.
"So the answer is no."