OT? How did they do the double image on the Patti Duke show?

Split screen:

Rita McLaughlin sometimes played the identical twin:

Look to see if there is ever any overlap. If the two images are always left screen/right screen with a gap between, then it is likely simple split screen. If it is done well, the only indication is an occasional shadow cut off at the 'dodged' [fuzzed] boundry. Early 'blue screen' had problems with the edges so it was usually fairly obvious.

Neil S.

This article is incomplete. Split screen was traditionally done in-camera, using a "matte box". This device -- which is basically a black-painted box resembling a rectangular lens shade -- sits in front of the camera and blocks half the image. Once the actor has performed one role, the film is rewound and the matte box flipped. The edge of the box is tapered in such a way that there is a gradual, calculated fall-off in light intensity at the inner edge, allowing the images to blend without an obvious "seam".

One of the most-famous uses of matte-box split screen occurs in Buster Keaton's "The Playhouse", where he appears nine times in the same scene!

It's also possible to create a dynamic split screen in an optical printer. This was most-famously done in "Bringing Up Baby", where we see Cary Grant leave Katherine Hepburn's apartment, followed (IIRC) by George, then Baby. As it would have been extremely difficult to choreograph/synchronize the movements of the animals, this scene was printed from three strips of film. If you look closely, you can see the join line moving across the image.

Digital technology makes split-screen effects almost trivial. The first time I saw a digital split screen was in an episode of ST:TNG where a character materialized from the transporter, then moved across the image without a cut. My jaw hung open for a few seconds, because I knew that wasn't possible in-camera. I then realized how it had to have been done.

Digital split screen is extremely flexible, because the dividing line can have any shape and be anywhere in the image, and can move or change shape from frame to frame.

Split-screen devices have been made for still cameras, most notably Polaroids.

There was "brightness key" technology in the days of black & white TV the precursor of color-key (commonly called blue screen, although green screen is also used). It was used for commercials on the Dinah Shore Show (among other things). It did require care in lighting the scene which was to have an insert. One of the comments about the technology was "Dinah Shore could have wound up with a Chevy in her mouth".

John Now realizing how many years I have spent in media technology...

Chroma key was what it was known as when color tv debuted.