What do in a Renaissance bedroom in Florence the ruins of a German castle and a canal in Venice have in common? That's the question that plagued OperaFest '86's set-designer, James Fouchard. His answer: arches.

OperaFest '86 (a collaboration among the University of Maryland, Catholic University and Montgomery College -- Rockville) presented Fouchard with the challenge of designing an interchangeable set for four relatively dissimilar operas: Puccini's "Gianni Schicchi" and "Suor Angelica"; Gilbert and Sullivan's "The Gondoliers"; and Strauss' "The Gypsy Baron." Budget restrictions and logistics required the operas to share one basic set design, and the all-purpose arch can embellish an interior scene or anchor an outdoor scene. One arch is a doorway; two, three or four arches are an arcade; a ruined wall, some draperies, a canopy bed or a few bookshelves give the arch a specific time and place.

"The real challenge," says Fouchard, "was to find a common-unit set that could adapt to the specific scenes of each opera. Not only do the geographical locales change, but so do the time periods."

"The Gondoliers" (tonight at 7:30 p.m.) and "The Gypsy Baron" run through March 8 at the Performing Arts Center of Montgomery College, Rockville Campus.