Acting for Two: Every single movie use this method. Of these movies, the only one that doesn't use Double Vision is Hibari's Lullaby (which avoids any shot in which the girls are both visible).
But I Play One on TV: These movies have convinced a fair number of people, mainly children, that Hayley Mills and/or Lindsay Lohan actually has a twin.
Fake American: Hayley Mills as both girls; one Bostonian, one Californian.
Fake Brit: Linsday Lohan as the English twin Annie. Hallie is an in-universe example as she tries to impersonate Annie.
Hey, It's That Guy!: Mary Kate is the girls' mother, while Pollyanna is apparently a twin.
In the remake Janice is a camp counsellor.
Hey, It's That Voice!: (1998) In Brazil, Hallie and Annie were, among many other roles, Emma Pillsbury, Heather, and freakin' Dora the Explorer.
Playing Against Type: Natasha Richardson at the time was known mainly for playing Ice Queen characters. This ended up being the role a good amount of people would associate her with.
Recursive Adaptation: The Hayley Mills Version had a Novelization of the Disney film version of the original novel, including a section of photos from the movie in the center of the book.
Star-Making Role: For Lindsay Lohan, though she starred in 2 Tv movies before following it up with her next hit .
Freaky Friday What Could Have Been: The technique used to film both girls at once was new in The Sixties, so Walt Disney was skeptical, and wanted two separate girls in the lead, until he saw how well it worked.