Periaktos

A periaktos (plural form periaktoi, from a Greek word meaning revolving) is a device used for displaying and rapidly changing theatre scenes.

It was first mentioned in Plato's Republic, in the story, "Allegory of a Cave" (c. 375 BCE), but its most intense use began in Renaissance theatre, as a result of the work of important theatrical designers, such as Nicola Sabbatini (1574–1654).

A series of periaktoi positioned one after the other along the stage's depth can produce the illusion of a longer scene, composed by its faces as seen in perspective.

A similar concept is used in some modern Trivision multi-message billboards, which are made up of a series of triangular prisms arranged so that they can be rotated to present three separate flat display surfaces in succession.

Early motion picture mechanical devices, such as the praxinoscope, were also based on rapidly rotating solid polygons, which had the successive animation or photographic plates affixed or projected to each face, thus providing the optical illusion of movement.