In the early days of the automotive industry, there was no way to tell if gasoline would knock in a new engine, or if a new gasoline would knock in any engine. The only way to find out if It would knock would be to fill up your tank and crank the engine. This problem was solved in 1926 by Dr. Graham Edgar of the Ethyl Gasoline Corporation, then a division of General Motors and Standard Oil. Dr. Edgar devised the octane scale by testing the knocking tendency of every compound that was similar to gasoline. He found that isooctane would not knock in any engine under any operating conditions, while n-heptane would always knock in any engine. By mixing isooctane and n-heptane in different amounts, he obtained fuels of all qualities, the percentage of isooctane in the mixture being the octane number. Commercial gasoline was compared to these mixtures in test engines to determine their octane number. When the scale was developed, commercial gasoline had octane ratings between 40 and 75, and the best were brought up to a rating of 87 by the addition of tetraethyl lead, making leaded gasoline. Today, due to environmental reasons, tetraethyl lead has been outlawed in the United States and many other parts of the world. However, we do not really need it, because new and better processes allow oil companies to make gasoline for our automobiles with octane ratings as high as 94.
From Frank A. Howard, Buna Rubber: The Birth of an Industry. New York: D. van Nostrand Company, Inc., 1947.