A bungee cord (sometimes spelled bungie), also known as a shock cord, (Occy strap / Octopus strap in Australian common usage) is an elastic cord composed of one or more elastic strands forming a core, usually covered in a woven cotton or polypropylene sheath. The sheath does not materially extend elastically, but it is braided with its strands spiraling around the core so that a longitudinal pull causes it to squeeze the core, transmitting the core's elastic compression to the longitudinal extension of the sheath and cord. Specialized bungees, such as some used in bungee jumping, may be made entirely of elastic strands.