Bunkers are typically subterranean, whereas blockhouses are mostly above ground. They were widely employed for weapons facilities, command and control centers, and storage facilities throughout World War I, World War II, and the Cold War. Bunkers can also be utilized as tornado shelters. Bunkers come in many forms, but they generally consist of a room with no more than 4 feet of clearance from the outside wall. They can be built into the side of a hill or mountain, under a house, or anywhere else that space allows. The walls should be at least 6 inches thick to withstand heavy artillery fire or nuclear attacks. Blockhouses are larger, better protected structures used to guard against attack by soldiers armed with muskets or cannons. Blockhouses often included several rooms with barricaded doors for defense. They were commonly used on frontier towns during the American Revolutionary War and the French and Indian War. Blockhouses remain in use today in places such as Afghanistan and Canada. There are three main types of blockhouses: picket, stockade, and earthwork. A picket blockhouse is simply a wooden post with a flagpole attached to it. The flag would either be plain white with red stripes (for peace) or blue with yellow stripes (for war). If the blockhouse was located on government property, then it would usually have one or more flags flying above it.