SWNTs, as the name would suggest, are made up of only a single layer of carbon structure in a solid sheet. Depending on how the sheet is wrapped, the resulting nanotube can be described similar to a vector (n,m), where n and m are unit vectors in two directions along the honeycomb structure of the sheet. If m=0, the nanotube is referred to as a "zigzag", as the exposed end of the tube will result in alternating points and dips. If m=n, the tube is called an "armchair", as the exposed end will show alternating high and low plateaus. Any other vector produces a tube that is referred to as "Chiral". Most SWNTs will have a diameter of about 1 nanometer but the tube length can be on the order of millimeters, millions of times longer than that of the diameter.
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MWNTs follow one of two models. In one model, multiple SWNTs of varying diameters are arranged concentrically within one another, similar to Russian Matryoshka dolls. In the other, a single sheet is rolled up on itself similar to a newspaper or an old scroll of parchment paper. In either case, the distance between layers is generally very small, about the same as that between layers of graphite in a pencil.