Once, it was an utterly radical idea. Only the most wild-eyed souls in the early 1900s thought that a woman who was doing the same job as a man ought to be paid the same amount.

But by the mid-1960s, the nation was mainstream enough to be made into law in both the Equal Pay Act of 1963 and the Civil Rights Act of 1964. By 1980, even the most conservative candidate said that equal pay for equal work was just plain American fair play.

Well, the irony is that this massive change in attitudes didn't actually change paychecks very much. In the late 1930s, the average workingwoman was earning about 58 cents for every dollar the average workingman earned. In 1980, she was earning 59 cents.

Women concept of equal pay for the same work doesn't help women, because few of them actually do the same work. About 80 percent of the workingwomen in this country -- 33 million of them -- work in jobs that are largely "women jobs." A full 99.5 percent of secretaries, 94 percent of telephone operators and 96 percent of registered nurses are female. About 96 percent of all career ladders are segeregated by sex and so are the paychecks.

The wages of these jobs are not primarily ruled on such economic niceties as supply and demand, or the mysterious workings of the free economy. One quick look at the Help Wanted pages makes that obvious. Despite the much-heralded secretarial shortage, wages in that field have gone up no faster than wages in the shrinking manufacturing market.

The wages of these jobs are kept low traditionally and arbitrarily because women do these jobs.

In response to this reality, the debate about women's work has changed focus. The quesion now being asked is whether women should get equal pay for "comparable work" or for work that is "equally valuable." The AFL-CIO has adopted this idea and so have many women's groups.

Until now, the concept has had an uncertain legal basis. Not long ago, nurses in Denver tried to sue for sex discrimination because they were paid less than tree-trimmers or parking-meter repairers. Their case was turned down by the circuit court.

Last week, in a careful, limited decision that virtually quivered with anxiety, the Supreme Court ruled 5-4 that, yes, employees can sue for discrimination under Title 7 of the Civil Rights Act even if they are not doing identical work.

The case in point was a classic. In Oregon, a group of jail matrons who guarded females was paid $200 less per month than a group of sheriffs who guarded males. The matrons didn't claim they held the same jobs. The women had fewer inmates and more clerical work. But an outside evaluator had shown the employer that the women were doing 95 percent of what the men were doing.

As Judith Lichtman, head of the Women's Legal Defene Fund, put it: "They were discounting the job because women did it."

Now these women, and millions of others, have won the right to sue. "This is the first setp," says Karen Nussbaum, president of Working Women -- A National Association of Office Workers. "It's saying, let's look at the job, and if your job is paid less because women to it, that's discrimination. Common sense tells us that; now the Supreme Court backs it up."

All of this new talk makes employers break out in hives. They talk about a flood of new cases. They talk about the cost of evaluating the relative work of a secretary and a truck driver. They warn us all about the dangers of a regulated economy. They even remind us about the Invisible Hand of the free marketplace, and how it works wonders for all of us.

But I suspect this Invisible Hand is the same one that sewed the Emperor's New Clothes. Working owmen already live in an economy that's regulated against them, a world in which parking-lot attendants make more than child-care workers.

With a little help from the courts and a lot of organizing, the concept of equal pay for comparable work may soon seem like just plain American fair play.