In the gimmick category, flaps have undoubtedly made a major contribution, enabling the realization of new effects impossible with simple manipulations.
However, this idea of trickery is by no means new! Without making it the first reference, traces of flap cards can be found as early as the 18th century in Récréations mathématiques et physiques.
For those interested, the same book provides this description of flap cards alongside the illustrations above. Find out how flap cards were talked about in the 18th century!
Over the years, this type of trick card has been perfected. Nowadays, there's even the addition of an elastic system for a visual change, and even a blocking system for advanced gimmicks, enabling the tricked card to be activated when desired. Nevertheless, it's interesting to note that the basic principle is not new!