A measure used to express the ability of gasoline to reduce engine knocking. Gasoline is a complex mixture of hydrocarbons: compounds containing hydrogen and carbon. Beginning chemistry students learn that "octane" is the name of a hydrocarbon whose molecules contain 8 carbon atoms and 18 hydrogen atoms, the 8 carbons being arranged in a long chain. The compound cars need to prevent knocking is not that octane but a different compound of 8 carbon atoms and 18 hydrogen atoms called iso-octane or, in the more precise language of chemical nomenclature, 2,2,4-trimethylpentane. (In an iso-octane molecule there are only 5 carbons in the chain. Carbons 6 and 7 are attached to the sides of the chain at the #2 position, and the last carbon is hooked onto the #4 position; chemists call this a branched hydrocarbon.) To determine the octane rating of gasoline, a sample of the gasoline is compared to a laboratory mixture of iso-octane and another hydrocarbon called heptane (heptane has 7 carbons and 16 hydrogens, with the 7 carbons in a chain). The mixture is adjusted until it has the same anti-knocking characteristics as the gasoline being tested. The octane rating is the percentage of iso-octane required in the laboratory mixture to produce this equality of knocking behavior. In fact, it's even more complicated: there are two ways to do the test, producing two ratings called the research octane number (RON) and the motor octane number (MON). The MON is typically 8 or 10 points lower than the RON for the same batch of fuel. In the U.S., the number posted on the gas pump is the average of the RON and MON; this average is called the pump octane number (PON). There is a similar unit, cetane number, used for rating diesel fuel. The octane rating is often misunderstood as a measure of the energy content of the fuel, but what it actually measures is the tendency of the fuel to burn rather than explode