First named in 1994 by Frederique Constant for their "Heart Beat" model, the open heart concept and terminology has spread throughout the industry. It is a limited form of skeletonization that focuses on the escape wheel and anchor (the most active parts of the movement) rather than other wheels.
Not all movements are amenable to open heart dials. The escapement must be visible through the top plate, either by design or through skeletonization of the plate. The escapement must also be oriented harmoniously to the overall dial layout, typically between 6:00 and 10:00 on the dial. Furthermore, complications like a date wheel can interfere with visibility so most open heart watches lack these.