Cylindrical fullerenes are known as carbon nanotubes. While C60 is a mix of five- and six-membered rings, nanotubes are usually arrays of only six-membered rings. They are just a few nanometers wide, but can be up to several millimeters long. The properties of this form of matter are almost unique in the world, and as a result carbon nan-otubes have caught the attention of many chemists. Nanotubes conduct heat and electricity very well, but are also extremely strong (specifically in tests of tensile strength).