Octane is a hydrocarbon found in petroleum. It has eight linked carbon atoms which may form a straight chain, various branched chains, or, in the case of cyclooctane, a ring. The straight and branched varieties share the same chemical formula and are therefore structural isomers. One highly branched form, 2,2,4-trimethyl-pentane, inaccurately named isooctane (which would be 2-methyl-heptane), has excellent anti-knock properties when burned in an internal-combustion engine, and its properties are used to calibrate the Octane rating of motor fuels. Isooctane is rated 100 on the octane scale and n-heptane, a hydrocarbon with very poor anti-knock properties, is rated 0.