The escapement in a mechanical timekeeper, whether a watch or a clock, regulates the transfer of energy from the power source, usually a mainspring, to the regulator. Most mechanical wristwatches use a lever escapement, originally invented by an English watchmaker in the 18th century but perfected by Swiss and American watchmakers in the following decades.
Though it operates with friction, meaning significant lubrication is needed, the lever escapement is precise, robust and self-starting, and importantly, easy to manufacture in large quantities. It’s found in almost all modern mechanical watches, having been refined to the point that advances in escapement technology are in materials like silicon, rather than the basic principles of its design and function. Examples of the lever escapement can be found in movements by Rolex, Breguet and Patek Philippe, among others.
Top image (foreground): Parts of an escapement, including a pallet fork and an escape wheel, by Rolex.