A rope bridge is a bridge constructed chiefly of rope. In its simplest form, it can be one or two ropes that bridge a river, enabling the traveller to be supported in their crossing and not be swept away. One rope above another, for feet and hands, may be referred to as a commando bridge. More complicated rope bridges can involve the use of multiple ropes, and boards as footpaths. One of the most complex examples of a culture using these as permanent, rather than temporary crossings, is the Inca civilization.