WHAT makes an artist choose relief?

"A kind of daring," says Hirshhorn Museum historian Judith Zilczer. Art that falls somewhere between the second and third dimensions, the 46 examples in "Relief Sculpture" at the Hirshhorn get you seeing that way, too. Among the rusty can collages and romantic portrait medallions here are also works in relief by Degas, Renoir, Picasso, Eakins, Sargent and Jasper Johns. No one said you had to be a sculptor to try it.

Relief sculpture heightens a statement, whether it's the anti-war message of David Smith's anti-medallions depicting grim deaths, or Red Grooms' tongue-in-cheek construction of a couple romancing on a Hollywood casting couch. Among these wall panels can be found milestones, too -- for example, the third architectural model for Rodin's unfinished masterpiece, "The Gates of Hell."

Zilczer calls that work "the single most important relief sculpture to be produced in 19th-century France." By the father of modern sculpture, it marked a transition from a traditional concept of relief as a sort of public monument in an architectural setting. The old relief might use figures from the Old or New Testament to flank a door.

"This subject was much more modern because it was more personal," says Zilczer. "It was intended as a representation of human suffering." Rodin's "The Thinker," which appears within the sculpture, would be one of several figures he developed that would later stand, or sit, on their own.

Not nearly as renowned, but important within this medium, is Alexander Archipenko, a Russian-born American whose delightful "Woman with a Fan, II" was assembled from colorful wooden shapes in 1915.

"He was one of the chief cubist practitioners of this hybrid form of relief sculpture," says Zilczer. One of the first to use color this way, Archipenko called it "sculpto-painting." The name didn't catch on, but the idea did.

To Johns, relief sculpture was another way of waving the flag: He'd painted flat flags thickly, and here is the same statement in bronze. Sparsely textured, the essence of flag- ness breaks through the picture plane with the barest outlines of a star field and stripes.

The idea of relief is stretched to its modern limits in John Okulick's trompe l'oeil "Full Moon Harvest." Like a compelling advertisement from the future, reaching out from the wall is a glistening harvest of aluminum bars stacked in four pine cubicles. An empty central chamber draws the eye to it.

And Louise Nevelson's modular black "Star Reflection" covers half a wall with wooden black holes writhing with black loops and polyps. What a relief that is.