How is a therblig defined?

Frank Bunker Gilbreth (1868-1924), the founder of modern motion study technique, called the fundamental motions of the hands of a worker "therbligs" (Gilbreth roughly spelled backwards). He concluded that any and all operations are made up of series of these 17 divisions. The 17 divisions are search, select, grasp, reach, move, hold, release, position, pre-position, inspect, assemble, disassemble, use, unavoidable delay, avoidable delay, plan, and test to overcome fatigue.

Gilbreth developed many of the concepts that became part of modern management techniques, and he also patented many inventions useful in the construction industry. One device he used in time and motion studies was his cyclograph or "motion recorder." An ordinary camera and a small electric bulb showed the path of movement. The light patterns reveal all hesitation or poor habits interfering with a worker's dexterity.