About 10 years ago I organized a field trip to a drywall factory. I took my employees, my father, the owner of the local drywall yard, and that’s about it. Turns out visiting a drywall factory is not at the top of most people’s list like it was on mine.
The most interesting thing to me was that after the gypsum core is poured and sandwiched between the two layers of paper the drywall has to set up (cool off) before it can be cut to the desired lengths. I believe that it is this process of pressing the paper faces flat with the liquid gypsum sandwiched between along the length of the drywall is what gives drywall its grain. Just like the grain in wood which gives wood much greater strength with the grain (typically the long direction) than against the grain. Drywall is approximately 3 times stronger in the long direction. Consequently, drywall hung perpendicular to the framing members is stronger than drywall hung parallel. (It is less likely to sag between joists).
Did you ever notice that the recommended on center spacing of ceiling joints is quite often closer when attaching drywall parallel to the joists? see chart below form USG Handbook:
Photo Above: JLC LIve Show: Demonstration on the strength of the lighter weight drywall, which is rated as more resistant to sagging than regular 1/2″ drywall. As a result the lightweight held a lot of weight and still did not break.