Carbon nanotubes consist of molecular cylinders of pure, hexagonally-arranged carbon atoms that resemble rolled-up sheets of chicken wire with a diameter measured in a few nanometres and a length of many microns. They occur in two main types, the single-wall carbon nanotube (SWNT) composed of a single cylinder of carbon , and the multi-wall version (MWNT) consisting of concentric tubes or cylinders of carbon (effectively straws within straws). The ends of the tubes are usually closed off by a carbon end-cap. Other variations on this theme include the double-wall tube, ‘herringbone’ and ‘bamboo’ structures.