Duffel bags, duffel bags or tool bags are large bags made of natural or synthetic fibers (usually canvas), historically with a drawstring top seal. Generally, military non-commissioned officers use duffel bags, which civilians use for travel, sports and entertainment. When used by sailors or marines, luggage is called a sea bag. The open structure and lack of rigidity of the suitcase make it suitable for carrying sports equipment and similar bulky items. Luggage bags are often confused with hoop-style hard-bottom zipper bags (often called gym bags).