In front of a Georgetown record shop, on a hot Friday night, the Metropolitan String Quartet had just finished a medley of Mozart. Three things immediately began to happen. Only two of them should have.

First came the applause -- loud billows of it from the crowd surging along the 3000 block of M Street. Then came the donations -- individuals stepping quietly up to an open violin case and dropping quarters or dollars into it.

And then came the uncertainty. Violist Uri Wassertzug began nervously scanning the crowd and the sidewalk. Although all four members of the quartet had licenses, and although the group had conspicuously posted a sign urging spectators to keep the sidewalk clear, the police might be back at any time.

Three times this summer, police have threatened to arrest the members of the Metropolitan for incommoding a public sidewalk. Even though the group sets up in the alcove of a private business, not on the sidewalk itself. Even though there have been no complaints from passersby. And even though the musicians obtained prior permission from the owner of the store in front of which they play.

Wassertzug and his fellow musicians (violinist Michael Barbour, violinist Jennifer Stephens and cellist Harriet Kaplan) are mystified. They are University of Maryland music majors, all around 20 years old, scrambling for a few extra bucks. They aren't bothering anybody. Is the D.C. incommoding law really designed to stop classical musicians who set up shop four feet away from a public sidewalk?

According to a police spokesman, it's a judgment call. The musicians might well be on private property, but if the crowds they attract are blocking public space, the police can order the musicians to cease and desist.

What it comes down to is the individual officer's judgment as to whether the sidewalk is blocked, or merely crowded, the spokesman said. In Georgetown, on a summer Friday night, that can be a tough call. But without clearer standards, one can hardly blame the policeman.

Let's give that policeman the tools he needs to do right by everyone, the members of a string quartet included. Displaying a sign urging that the sidewalk be kept clear, as the Metropolitan does, should be enough to avoid arrest. Without such a change in the law, the Metropolitan is going to set up shop elsewhere -- and Georgetown will be poorer for it.