Houses in medieval towns were built mostly of wood, and people heated their homes and cooked over an open hearth fire. If a fire spread at night, an entire town could be reduced to ashes. So as evening fell, a bell would ring, and at that signal everyone had to bank their hearth fire under ashes and put it out. In Old French this command was cuevrefeu — couvrir ("to cover") joined with feu ("fire"), meaning "cover the fire." After the Norman Conquest the custom crossed into England and became curfew. But the bell that told people to cover their fires was also a signal to go home and rest. Over the centuries the fire-safety meaning faded, and only the sense of a ban on going out after a set hour remained.