14 CFR § 121.440

No certificate holder may use any person nor may any person serve as pilot in command of an airplane unless, within the preceding 12 calendar months, that person has passed a line check in which he satisfactorily performs the duties and responsibilities of a pilot in command in one of the types of airplanes he is to fly.

A pilot in command line check for domestic and flag operations must—

Be given by a check pilot who is currently qualified on both the route and the airplane; and

Consist of at least one flight over a typical part of the certificate holder's route, or over a foreign or Federal airway, or over a direct route.

A pilot in command line check for supplemental operations must—

Be given by a check pilot who is currently qualified on the airplane; and

Consist of at least one flight over a part of a Federal airway, foreign airway, or advisory route over which the pilot may be assigned.