On a set of doubles or triples, which shut-off valves (cut-out cocks) must be in the CLOSED position?

AAMVA CDL Manual, Section 6.2.7, p. 6-6

Getting this backwards can cause a loss of braking or a false low-air warning while driving a set of doubles or triples. A driver who leaves the last trailer's valves open, or closes valves further up the line, can disrupt air flow to trailers that are actually being towed. Examiners test this because it's a quick pre-trip check that prevents a serious on-road brake problem.