a 2x6 frame at 24-inch centers with single top plates, two stud corners, no jack studs, no cripples and single headers (and in many cases no headers at all).
It is cheaper and faster to build and saves energy. What is not to like? It is cheaper because it uses 5 to 10 percent less lumber (board-feet) and it is faster because it uses 30 percent fewer pieces. It saves energy because it provides a 60 percent deeper cavity (which allows 60 percent more cavity insulation) and because it reduces the framing factor from 25 percent to 15 percent.