 There is no official definition of a skeleton clock "per se", but a major portion of the main parts of the timepiece should be openly visible from the front of the clock and most often from the back as well in order for it to be considered and accepted as a skeleton clock.The parts most commonly showcased by a skeleton design are those exhibiting either the most movement or the most attractive design.These may include, but are not limited to the escapement, balance wheel and balance spring, mainspring, and tourbillon.