WHEN Sandra Day O'Connor joined the Supreme Court 10 years ago this month, an exercise break for clerks meant playing basketball on the roof - the "highest court in the land."Justice O'Connor sponsored a drive to bring in an aerobics instructor for the women working at the court. But the first and still the only woman in American history to sit on the Supreme Court, a sister among the brethren, has not wrought any revolutions in her decade on the bench. The more serious effects of Justice O'Connor's barrier-breaking gender are much more subtle and workmanlike. As a judge, Mrs. O'Connor is practical and commonsensical more than big-picture intellectual. She is conservative, but the rightward shift of the court in recent years has put her near the middle. She is independent-minded, less predictable than some, but also a bridge builder between justices. She has been watched intensely in recent years for signals on abortion, and when a case comes this year or next that addresses it directly, her vote will be pivotal. One close analyst of O'Connor opinions, Susanna Sherry of the University of Minnesota law school, sees a consistent underlying approach that she identifies as feminine. Nothing so blatant as aerobics classes, however. On O'Connor's role as the court's first woman: "Frankly, I did not see that come into play," says one former clerk, in a comment echoed by several others. Asked to describe her most distinguishing characteristics, her former clerks are as likely to cite her background as a state legislative leader as her gender. But O'Connor is also clearly aware of her historic role. She likes to recount that the attorney general who summoned her to Washington to discuss her Supreme Court appointment in 1981 worked for the same law firm that once offered her a job as a secretary - after she had graduated second in her 1952 class at Stanford Law School. Justice O'Connor grew up on an isolated ranch in Arizona and built a career in state politics. She rose to the rank of majority leader of the Arizona Senate before her appointment to the state Court of Appeals. Unlike many Supreme Court justices, she did not arrive with a background in Constitutional law. In her first year or two, she was noted for her diffidence in questioning during oral arguments. But her confidence, as it showed in her questioning of lawyers, grew steadily. Her discipline is prodigious. Clerks recall that she worked 10-hour days, taking reading home, and six-day weeks. "The hallmark of her preparation was just an incredible thoroughness," one former clerk says. "She got more pre-argument preparation than possibly any other justice," he adds, explaining that she never let a case slide past by following another justice's lead. "All of us in those chambers were acutely aware that she was the first woman and were doubly careful," says Peter Huber, who clerked for O'Connor during the early 1980s. In public, O'Connor is as disciplined about keeping her profile low as she is about case preparation. She rarely releases texts of speeches, for example, or grants interviews. The character of her decisions has established some visible patterns, however. "Justice O'Connor is less burdened with intellectual baggage than most justices on the court," says another former clerk who, like many interviewed for this article, wanted his name withheld. She may also be the strongest federalist currently on the court - deferring to state governments unless clearly required not to by Congress or the Constitution. Her strongest stamp on the court as a whole, however, is developing in two areas of legal doctrine: establishment of religion and affirmative action. For two decades, the court's doctrine on the separation of church and state has centered around the three-part "Lemon test." To pass constitutional muster, a government statute or activity must have a nonreligious purpose, must not have the effect of advancing or inhibiting religion, and must not excessively entangle government and religion. The court is leaning toward a less strict test of separation. Some justices, such as Antonin Scalia, are ready to accept any statute that does not coerce religion. O'Connor is not. She defined a new test in the 1984 Lynch v. Donnelly case. She read precedent to forbid government endorsement or disapproval of religion because: "Endorsement sends a message to nonadherents that they are outsiders, not full members of the political community...." Many court-watchers believe her reasoning may carry the court in an establishment case to be decided this session about prayer at a public-school graduation. Her rationale in Lynch carries a subtle but fundamental shift of view, notes Professor Sherry, and one that fits a pattern of what she calls feminine jurisprudence. In her Lynch opinion, O'Connor moved away from the language of rights that permeates modern American politics and legal thought to a language of membership and community. At root, it is a language not of individualism but of relationships. If this emphasis is generally more feminine than masculine, says Sherry, so is O'Connor's emphasis on the concrete facts of each case over abstract ideology. So is her emphasis in criminal law on whether verdicts may have been in error over cases that appear strictly procedural violations, Sherry says. O'Connor seems to ask, says Sherry: "If you have a credible case that you were wrongly treated then we will hear you." O'Connor prefers to keep her decisions in the context of the facts of a case, rather than creating bright-line rules, says Sherry. Finally, Sherry adds, "she plays the role of peacemaker and mediator on the court." She often writes concurring opinions with the majority that reaches out for common ground with dissenting opinions. Some of this conciliation may derive from her legislative background. During the term Glen Nager clerked for her, he recalls, "She made an unusual effort to talk to other justices." She would take the draft of an opinion to a justice considered a swing vote, for example, to see if he would agree. On affirmative action, O'Connor wrote an opinion in Crosson v. Richmond in the 1988 session that narrowed the scope of permissible set-aside plans. But it was the first affirmative-action opinion case where a court majority signed one opinion, says Sherry. On the bench, O'Connor's tone sometimes registers a stark contrast to those around her. In oral arguments last week, Justice Anthony Kennedy had confounded a young lawyer arguing before the bench with a hypothetical question that left the attorney momentarily off-balance and groping for a response. Justice O'Connor entered her own question - no less challenging - but with a calm, reassuring manner like a respectful parent. It almost seemed to help the lawyer find his balance again, in time for a crisper, more abrupt challenge from Justice Scalia.