Prior to 1950, homes didn’t have sheetrock. They had lath (strips of wood nailed to the studs) and plaster- a mixture of lime, sand and water that was spread on the wall by hand. After the plaster dried (cure time was about a month), it created a reasonably smooth, hard surface. The use of nails to hang pictures was discouraged since this caused the plaster to chip. Paint and wallpaper were the usual coverups. Fast forward to today- homeowners are choosing to strip off the these layers and keep the look of raw plaster.