By LINDA GREENHOUSE

WASHINGTON -- During her 18 years on the Supreme Court, Sandra Day O'Connor has often been the justice in the middle of a sharply divided court. In Monday's 5-to-4 decision on sexual harassment in the schools, she was, perhaps more clearly than ever, the woman in the middle as well.

There was a memorable scene, lasting no more than 10 minutes, as O'Connor and Justice Anthony M. Kennedy each summarized their respective majority and dissenting opinions for a courtroom audience of lawyers and tourists. The two justices, both Westerners in their 60s, both Stanford University graduates, both the appointees of Republican presidents, each the parent of three grown children, might have been speaking from different planets.

For O'Connor, explaining the majority's view that public school districts can be held accountable for one student's flagrant sexual harassment of another, the case was about sex discrimination so severe as to destroy the learning environment in the classroom. For Kennedy, speaking for the four dissenters, the case was about federal intrusion into a place where the federal government has no business.

That O'Connor and Kennedy are longtime allies in the cause of states' rights made this non-meeting of the minds all the more striking. Ever since O'Connor arrived on the court in 1981 as the FWOTSC, as the First Woman on the Supreme Court has drily referred to herself, the obvious question has been: does it make a difference? Monday's decision, indeed O'Connor's entire Supreme Court career when viewed through the lens of gender, suggests that it does.

That is not a notion completely foreign to O'Connor herself, who wrote, concurring in a 1994 decision that made it unconstitutional to remove prospective jurors on the basis of sex, that although there were no "definitive studies" on how jurors behaved in cases of sexual harassment, child custody, or domestic abuse, "one need not be a sexist to share the intuition that in certain cases a person's gender and resulting life experience will be relevant to his or her view of the case."

Nor is it completely foreign to her junior Supreme Court colleague, Ruth Bader Ginsburg, who joined her opinion this week in Davis vs. Monroe County Board of Education. Ginsburg recounted in a 1997 speech to the Women's Bar Association here how a year earlier, as she announced her opinion declaring unconstitutional the all-male admissions policy at the Virginia Military Institute, she looked across the bench at O'Connor and thought of the legacy they were building together.

Ginsburg's opinion in the Virginia case cited one of O'Connor's earliest majority opinions for the court, a 1982 decision called Mississippi University for Women v. Hogan that declared unconstitutional the exclusion of male students from a state-supported nursing school. O'Connor, warning against using "archaic and stereotypic notions" about the roles of men and women, herself cited in that opinion some of the Supreme Court cases that Ruth Ginsburg, who was not to join the court for another 11 years, had argued and won as a noted women's rights advocate during the 1970s.

Addressing the women's bar group, Ginsburg noted that the vote in O'Connor's 1982 opinion was 5 to 4, while the vote to strike down men-only admissions in Virginia 14 years later was 7 to 1. "What occurred in the intervening years in the court, as elsewhere in society?" Justice Ginsburg asked. The answer, she continued, lay in a line from Shakespeare that O'Connor had recently spoken in the character of Isabel, Queen of France, in a local production of Henry V: "Happily a woman's voice may do some good."

"I don't like the argument that we have to have women or else nobody will listen, but it does seem to be making a difference," Suzanna Sherry, a law professor at the University of Minnesota, said in an interview Tuesday. She said that while it was always clear that O'Connor had first-hand experience with the classic model of sex discrimination, "the question has been whether she would recognize the changing face of discrimination, and it looks like she can."

Another scholar of the Court, Nancy Maveety, a political scientist at Tulane University who wrote a book about O'Connor, "Strategist of the Supreme Court," (Rowman Littlefield, 1996) also said she found O'Connor's vote this week notable. "I would have expected a fence-straddling concurrence," Maveety said. She said the outcome was an important indication of the relative weight O'Connor gave to the discrimination arguments as opposed to the federalism concerns of the dissenters. "For something to trump federalism for her, that's a big deal," Maveety said.

A top graduate of Stanford Law School in 1952 (her classmate, William H. Rehnquist, went on to a Supreme Court clerkship, an opportunity that was not then open to women), Sandra O'Connor applied to law firms only to receive job offers as a secretary. The experience led her into the public sector and eventually to elective politics. She served as majority leader of the Arizona State Senate in the early 1970s, the first woman in the country to hold so high a state legislative office, and from there became a state court judge.

Ruth Bader Ginsburg, a top graduate of Harvard and Columbia Law Schools, also found doors closed to her, although her career led her to law school teaching and advocacy, for some years under the aegis of the American Civil Liberties Union. Their politics and their views on many issues, perhaps most notably on the appropriate role of race in political districting and government contracting, are significantly different. But there is clearly a bond between the two justices. They laugh about the fact that some of the most experienced lawyers in the country mix up their names when addressing them during argument sessions.

Some court observers think the presence of two women on the court is substantially greater than the sum of one plus one. "Having two makes a huge difference," said Peter J. Rubin, a law professor at Georgetown University who clerked on the court in the early 1990s, just before Ginsburg's arrival.

"It just makes it a lot easier," Rubin said. "It's only human nature. You're in an insulated environment. You're not out there on the ground. When two colleagues who have had different experiences say that here is a problem to be taken very seriously, it's bound to have an impact."