At Backstage Inc., all the store's a stage. Characters appearing at the new performing-arts store might be actors, members of a stage crew or of the audience, since the shop offers virtually everything from the nails for the stage floor to scripts for hundreds of plays to the playbills for Broadway shows. Stage designer Jean Rosenthal stars in the Backstage tale. Simply, she was tired of going to umpteen little stores to collect materials for every show she worked on; one place for the lighting needs, another for costume design, another for makeup. . . Enter Backstage. The three rooms of the large studio even resemble the black box of a darkened theater. The high ceilings are gray, as are the wooden floors, lighted, from high above, by rows of stage lights (not for sale; the only things not for sale here). The most obvious thing the store sells is posters. Framed and unframed, old shows and new shows, commercial and rare. The next act is the library: Four walls of books, on the history and philosophy of theater, volumes of the 1918 Theater magazines, great big glossy hardbacks about Broadway and Hollywood, Cliff Notes for plays. The director ordered lots of books, and she got lots of books. In the room with the semicircle of bright lights on the wall, under the wigs and elaborate handmade acrylic mache masks, is the theatrical makeup. Jars, tubes, pots of color at the end of the rainbow. And there's more: sailors' caps ("Dames at Sea?"), the musical scores of dozens of shows, Altman wrenches. Altman wrenches? "They're for hanging and focusing stage lights. For stage crews they're like a hammer to a carpenter," Rosenthal explains to someone whose attention and interest have already turned to stacks of pages torn from Victorian women's magazines depicting 19th-century apparel. On cue, Rosenthal explains, "And those are good for costume reference. Otherwise you have to spend hours in antique shops or libraries." If this is such a great idea, why hasn't anyone done it sooner? "I beat them to it," Rosenthal says cheerfully. Given a push when the Kennedy Center made its debut a decade ago, Washington's performing-arts community has "pulled together" now, says Rosenthal. It was all in the timing, a tribute to her mind for business that complements the theater in her blood. Her parents have had more than bit parts in the whole show, passing down not only their love for the theater but financial help and parent-power behind the counter.

BACKSTAGE INC. 2101 P Street NW. 775-1488. Mondays through Saturdays, 10 to 7.