“They were very, very good friends,” said Donald Judd’s son Flavin in 2016. “Don judged people by their artwork, so Dan Flavin was pretty far up there.” Dan Flavin and Donald Judd met in 1962. It was at an apartment in Brooklyn, where artists had gathered to discuss the founding of an artist-run gallery. After that, as the painter John Wesley described, “the two became Flavin and Judd for a while. The two names were together.” A year later, they exhibited their work in an exhibition at Richard Bellamy’s Green Gallery, and Flavin fixed his first fluorescent light tube to a wall. The two men collaborated on pieces, and remained close. In this monumental exhibition, work by these modernist pioneers are placed side by side. —Elena Clavarino