“Downton Abbey,” the most-watched drama in PBS history, is about to hang up its flapper dresses and maids’ aprons, and put away the dangling earrings, jeweled headpieces and chandeliers. The series, which has become a global phenomenon, airs its 90-minute swan song Sunday night on “Masterpiece.” Through six seasons, the show has followed the aristocratic Crawley family and its dwindling ranks of servants from the rule-bound Edwardian era through World War I and the Roaring Twenties.

Before we bid farewell, here are some burning...