Open heart - this complication is simple, a hole is cut in the dial over the escapement so that people can see the heart of the watch "beat."
Perlage - A pattern of small overlapping circles applied to the structural components of a watch movement and other surfaces like the rotor. The pattern is created using a rotating wooden peg.
Pinion gear - This part is a shaft with a gear attached. The gear engages other parts of the gear train. As the gear train moves, the shaft rotates, powering watch hands and other parts. A common name for the pinion that powers the hours, minutes, and seconds hand is cannon pinion.
Power reserve - this complication is the "gas gauge" of automatic watches. Like a flyback complication, a hand indicates how much power the watch has left before it stops. The most common markings are in hours.
Skeletonization - When a watchmaker skeletonizes a watch, they remove as much of the material from the movement as possible without weakening the structural integrity of the watch. Skeletonized movements usually receive additional engraving, and the gears and other components of the movement may be upgraded appearance-wise now that they are visible. Dials are optional for this complication, which requires installing the indices differently.
Subdial - A smaller dial on the main dial. These are used for a variety of purposes, such as displaying elapsed time in chronograph watches, day of the week, day of the month, moon phase, and more. They can be recessed or flush with the main dial, depending upon the effect the watch company wants to create.