Applicable U.S. Federal Regulations
The federal aviation regulations applicable to the design and operation of commercial aircraft are contained in "Title 14 - Aeronautics and Space" of the Code of Federal Regulations (14 CFR). Part 25 of 14 CFR provides the airworthiness standards for transport category airplanes. Part 121 of 14 CFR contains the operating requirements for air carriers and commercial operators. Part 125 of 14 CFR covers (i) the certification and operations of aircraft having a seating capacity of 20 or more passengers OR a maximum payload capacity of 6000 or more pounds and (ii) the rules governing persons on board such aircraft.
The current and full text of 14 CFR can be found on the U.S. Government Printing Office website