Sanforized Flannelettes - now 32% less incredibly flammable than before! “Sanforization” was a process that kept cotton from shrinking, one of those miracles of science. Rather low-tech, from the wikipedia description:

The cloth is continually fed into the sanforizing machine and therein moistened with either water or steam. A rotating cylinder presses a rubber band against another heated rotating cylinder, thereby the rubber band briefly gets compressed and afterwards shrinks to its final size. The cloth to be treated is transported between rubber band and heated cylinder and is forced to follow this brief expansion and recontraction and thus gets shrunk.

Got that? It’ll be on the test.