When pulling a set of doubles with trailers of different weights, where should the more heavily loaded semitrailer be positioned for the safest handling on the road?

49 CFR 383.115(b) ("Proper placement of heaviest trailer"); AAMVA CDL Manual Section 7.2.1 "Coupling Twin Trailers" ("Secure Second (Rear) Trailer" subsection), p. 7-1, Version July 2017

This single rule is a guaranteed knowledge-test topic because it is the entire content of its own CFR subsection - and it directly affects crash risk, since the wrong arrangement makes the rig far more prone to the rearward amplification that causes a rear trailer to roll over.