WHEN THE SUPREME Court decided the Miranda case a decade ago, we thought it was saying that people are entitled to know what their constitutional rights are before they are asked to confess to a crime. That seemed to us to be logical and fair.

But now it is turning out that the Court, at least in its present incarnation, has something else in mind. In a decision last Tuesday, it brushed aside that reading of Miranda. It now says that people have to be told what their rights are only after they are "in custody." By interpreting the decision in this narrowest possible way, the majority can claim that it has not reversed Miranda. But that decision's basic rationale is gone, its effectiveness has been diluted, and large ambiguities have been created. (When, precisely, was the suspect placed in custody?) And, as if to show its disdain for the whole issue, the Court did all this summarily, without benefit of briefs or arguments.

The Miranda decision, you will recall, was one of the highlights of the years during which Earl Warren was Chief Justice. It was the last in a long series of cases in which the Court curtailed the growing tendency of police to rely more on persuading suspects to confess than on seeking other evidence to convict them. It was denounced immediately by some but not all law enforcement people as an unmitigated disaster and was exploited by one presidential candidate - as part of his campaign. Most police departments, as far as we can tell, have adjusted admirably to Miranda's requirements with relatively little loss of efficiency. But now the trend could shift back. The police appear to be free to question suspects once again, without advising them of their rights - as long as the arrest has not been made.

There is, we might add, one interesting possibility that the Court's ruling will have a small effect - at least in certain states. Justice Thurgood Marshall, in dissent, noted that state courts are free to interpret their state constitutions to bar what the Supreme Court has said the U.S. Constitution does not bar. One of them did just that lately - in South Dakota. That state's supreme court is not alone in its efforts to preserve some of the advances in criminal procedures that the current membership of the U.S. Supreme Court seems intent upon overturning. Perhaps others will join it in the area affected by the Miranda rulings. It hardly seems unreasonable to require that suspects be told their rights before they are asked to send themselves to jail.