From costumes that have enough stretch for a flurry of high-kicks to flights of stairs both sturdy enough to support a dozen actors and light enough to be lifted high above the stage for storage, theatrical designs improve upon items we mortals use every day. But those of us in the audience rarely see how these designs come into being; we only see the results from seats in the second mezzanine. The Library of Congress offers some insight into the magic usually reserved for Broadway’s backstages in its new exhibition, “Grand Illusion: The Art of Theatrical Design.” Learn the secrets of theatrical design by consulting notes from George Gershwin and John Kander, see a scale model of the set for Grand Hotel, and scour documents for insight into how theatrical designs have changed over the past century. You won’t get to try on any costumes from Chicago, but use your imagination and leave the black fringe for the kickline. The exhibition is on view Mondays through Fridays, 8:30 a.m. to 5 p.m., to July 25, at the Library of Congress James Madison Memorial Building, 101 Independence Ave. SW. Free. (202) 707-8000. loc.gov.