Joan Acocella for The New Yorker

For twenty years, the Mark Morris Dance Group, like almost all other modern-dance companies in the arts-unfriendly United States, has been operating out of rented studios, with no showers, not to speak of dressing rooms. The company's offices, too, were rented, and crowded, with boxes of wrinkled costumes piled up next to the desks. (Morris himself had no office. He operated off his dinner table.) The M.M.D.G. has long been one of the most esteemed dance troupes in this country and abroad, but it lived like a band of itinerant players. Now, as the new building attests—and as the three-week, five-program, twentieth-anniversary season that the company opened last week at BAM demonstrates even more clearly—it is an institution.