The Bobbsey Twins are the principal characters of what was, for many years, the Stratemeyer Syndicate's longest-running series of children's novels, penned under the pseudonym Laura Lee Hope. The first of 72 books was published in 1904, the last in 1979, with a separate series of 30 books published from 1987 through 1992. The books related the adventures of the children of the middle-class Bobbsey family, which included two sets of fraternal twins: Bert and Nan, who were 12 years old, and Flossie and Freddie, who were six.