Trivia

For most of the scenes featuring Patty and Cathy, Rita Walter played "the back of either Patty's or Cathy's head", as appropriate. She can also be seen in several episodes as a background character.

Patty Duke was only sixteen when production began, and thus it was decided to film the show in New York City at the Chelsea Studios in Manhattan. New York's child labor laws were more liberal than California's--especially that state's Coogan Law (named after child actor Jackie Coogan), which regulated the working conditions of child actors. Filming in New York City allowed Duke to work more hours per day, an advantage since she pretty much carried the show. Duke turned eighteen during filming of the final (1965 to 1966) season, and although the season started in New York City, the whole production was moved to Hollywood by season's end much to the chagrin of Duke, who wished to stay in New York City. Nevertheless, the setting of the series remained the same Brooklyn Heights neighborhood it had always been.

William Schallert, who played Martin Lane (Patty's father), also played Kenneth Lane (Cathy's father and Martin's twin brother) during the Christmas episode where Martin was forced to fire Kenneth.

Bette Davis, then playing a double role in the film Dead Image (1963), was approached to play a guest spot. She demanded too much money and too many perks, so they decided to drop the idea.

Although the series was still doing well enough in the Nielsen ratings, ABC decided not to renew it for a fourth season. ABC wanted all of their black and white shows to switch to color production. United Artists wanted more money to make the change. The network decided it would be more cost-effective to develop a new color show instead.