A hammock is a type of bed or couch made of canvas, netted cord, or strong cloth that is hung between two supports at each end. It is typically used for swinging, sleeping, or resting, and is often seen as a symbol of summer, leisure, relaxation, and simple living. Hammocks were developed by native inhabitants of the Americas for sleeping, and later used aboard ships by sailors to enable comfort and maximize available space. They were also used by explorers or soldiers travelling in wooded regions and eventually by parents in the early 1920s for containing babies just learning to crawl. Today, hammocks are popular around the world for relaxation and are also used as a lightweight bed on camping trips. Some types of hammocks offer a ridgeline to make set up easier, and may also include a slit for entry through the bottom of the hammock. Special webbing straps (called "treehuggers") are used to loop around the supports to hang the hammock.