A standard laboratory measure of a fuel's ability to resist knock during combustion in a spark-ignition engine. A single-cylinder four-stroke engine of standardized design is used to determine the knock resistance of a given fuel by comparing it with that of primary reference fuels composed of varying proportions of two pure hydrocarbons, one very high in knock resistance and the other very low. A highly knock-resistant isooctane (2,2,4-trimethylpentane, C₈H₁₈) is assigned a rating of 100 on the octane scale, and normal heptane (C₇H₁₆), with very poor knock resistance, represents zero on the scale. Octane number is defined as the percentage of isooctane required in a blend with normal heptane to match the knocking behavior of the gasoline being tested.