"Relief Sculpture: Selections from the Museum's Collection" is another of those little shows that make the Hirshhorn different: It is a leveling exhibit. It is broad-gauged, rhyme-filled modest. It includes some mighty names -- Eakins, Degas, Rodin, Jasper Johns, Picasso -- but nothing you would call a masterpiece of art.

Some viewers yearn for masterworks. This show will disappoint them. Others are content to wander in that telling zone between art's peaks and valleys. They will find this show a treat.

The founder of the Hirshhorn, or so his critics claim, was far too fond of bargains. He dodged big-ticket items, and fell for minor talents, and padded his collection with all sorts of lesser objects -- sketches, minor paintings, little desk-sized sculptures -- by both big and lesser names. But turn that argument around: What some see as a defect may have been a virtue, too.

Joseph Hirshhorn's greatest gift was his gift for open-mindedness. The Hirshhorn is in consequence the least snobbish of museums. The collection it displays is exceptionally instructive because it's exceptionally inclusive. Masters sometimes trip. Lesser mortals sometimes shine. The Hirshhorn rightly teaches that the history of art is more than just a hit parade. Look, for instance, at this show.

The Renoir on display is a work of small importance. So too are these early, rather preachy David Smiths. But there are first-rate things on view here by artists less well known -- Saul Baizerman, for instance, or Medardo Rosso. That little Degas bronze of women playing in an orchard reminds us of his mastery, as do those Picassos. Of the 46 reliefs on view, very few are losers, almost all are well worth seeing, but the spirit that connects them is nicely democratic. No one steals this show.

Its levelness in curious ways contributes to its usefulness. These objects sing in concert, supporting one another. Together they reveal something worth remembering about how artists think.

The idea of the bas-relief is, of course, not new. The artists of the Ice Age who chose to paint their bison on bumps found on cave walls understood it well. So too did the Assyrians, and Greeks who carved the noble friezes on the Parthenon, and the sculptors who put emperors on Rome's imperial coins. Statues may be rounded, paintings may be flat, but what's wrong with a compromise? Why not make a painting that juts into the room, or a sculpture that insists on just one point of view? Reliefs help build a bridge between two and three dimensions. Not quite pictures, not quite statues, they're a little bit of both.

Artists love such blurry zones. The 19th-century painters whose still lifes of dead game were so skillfully contrived that the dead birds and dead rabbits almost seemed the real thing were not really hoping to utterly deceive us. Nobody expects to find a bleeding rabbit in a frame above the sofa. It is not true deception but flirtation with deception that lends such works delight. Or consider Andy Warhol, whose early "boring" films -- "Eat" or "Sleep" or "Empire State," in which the camera stayed still and almost nothing happened -- were poised with such precision between the movie and the still.

Modern art is full of play, retrieval and experiment. So too is this show. Is it possible to mix 19th-century naturalism and the style of the ancients? Philadelphia's Thomas Eakins, represented here by a bas-relief in bronze of "A Youth Playing the Pipes," gives it a good try. Is it possible to wrestle the classical tradition into something wholly new? When, in 1880, Auguste Rodin made this plaster sketch for his "Gates of Hell," he was trying to do just that.

How flat can a sculpture be and still be a sculpture? "Flag" by Jasper Johns offers one sly answer. It is a copy of a painting, but a copy cast in metal. Nothing makes it sculptural save the thickness of the waxy paint from which this bronze was cast in 1960. Its material says, "I'm sculpture"; its flatness says, "I'm not." The Baizerman, in contrast, resembles a traditional statue in the round, but a statue cut in two. A life-size image of a mother with a child at her breast, it was hammered by the artist from a single sheet of copper. Somehow it combines a yearning for tradition with a distinctly 20th-century kind of technical confession. When, inevitably, the viewer peeks around its edges and finds supporting steel struts in the hollowness behind, the statue seems to say, "I'm not entirely old-fashioned; look how I was made."

A number of these objects are political in spirit: The muscled heroes we discover in this handsome bronze relief by the Belgian sculptor Constantin Meunier (1831-1905) are brickmakers at work. The early David Smiths on view, his "Medals for Dishonor" (1939-40), are rather silly, although heartfelt, prophesies of war; the one on "Propaganda" includes a bulky radio rushing into battle on pudgy little legs. Some of these sculptures mourn. "Rest in the Peace of God's Hands," a bronze grave relief cast by Ka the Kollwitz in 1936, urges us to sob. Red Grooms' "Hollywood," in contrast, a scene of glamor and seduction, urges us to grin.

The show begins by raising a single formal question -- What is relief sculpture? -- but it does not do so for long. It touches on religion, politics and play, and leads us all the way from the heritage of Greece to the old junk found in attics. That naked youth by Eakins, and that abstract wooden-wall relief made by Louise Nevelson in 1972, are not tied to one another by a single highway-of-art historical development, but by a maze of byways. This show explores them all.

When one leaves the exhibit, that huge new object by Frank Stella and the quarters in one's pocket seem suddenly allied. Joseph Hirshhorn had his faults, but he understood connections. It is not easy to imagine another art museum that could have drawn from its own holdings an exhibit so inclusive, unpretentious and instructive. It closes April 13.