An ultra-thin watch is a watch whose size and thickness have been
reduced to an absolute minimum. Watchmakers consider the bulk of
the work to be in the miniaturization of the parts, rather than
in actual complicated watch work.
In 1903, JLC designed a 1.38 mm thick pocket watch movement.
In 1925, AP made a 1.32 mm pocket watch movement, Blancpain made
a 1.75 mm thick wristwatch.