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In a film crew there are two kinds of best boy: best boy electric and best boy grip. They are assistants to their department heads, the gaffer and the key grip, respectively. In short, the best boy acts as the foreman for his/her department.
Job responsibilities 
Best boys are responsible for the day-to-day operation of the lighting or grip department. Their many responsibilities include the hiring, scheduling, and management of crew; the ordering, inventory, and returning of equipment; overseeing workplace safety and maintaining discipline within their department; completing timecards and other paperwork; stocking of expendables; loading and unloading production trucks; planning and implementing the lighting or rigging of locations and/or sound stages; coordinating with rigging crews and additional photography units (if applicable); handling relations with the other production departments; overseeing the application of union rules (where relevant); and serving as the day-to-day representative of the department with the unit production manager and coordinator of the film.
The best boy also commonly accompanies or stands in for the Key Grip or Gaffer during technical scouts. During shooting, the best boy may also cover for the key grip or gaffer when he/she is on break or otherwise away from set.
On films with very small crews, the electric (lighting) department often consists of only a gaffer, a best boy, and a few electricians. The grip department may include only a key grip, a best boy, and a few grips. Large-scale productions such as major motion pictures commonly include full-time rigging and second unit crews, and in total may hire many dozens of grips or electricians at one time.
The term predates the film business. In the English apprentice system, the "best boy" was the Master's oldest, most experienced apprentice, who thus had more responsibilities than the others. In effect he was the second person in charge, as a best boy grip or electrician is today, carrying out the key grip's or gaffer's instructions.
As the gaffer is sometimes credited as the chief lighting technician, the best boy electric is sometimes credited as the assistant chief lighting technician.
In the early days of film, the key grip would ask the gaffer, "can I borrow your best boy for an hour?" and vice versa. The term stuck.
The title is accepted for use in credits by the BBC.
Usage outside of English 
Many French language films made in Canada use Best Boy in their credits. The term has been known to appear in the credits of some French films made in France, but it has not been known to appear in French, Belgian or Swiss films.
The German film Auf der anderen Seite (The Edge of Heaven), which takes place in Germany and Turkey, uses the term Best Boy in its German credits.