Aviation gasoline continued to develop and obtain increasingly higher octane ratings through the end of World War II. Since that time about 6 grades have seen service. The table below shows various current and historic grades. The advent of jet engines and the subsequent removal of gasoline powered aircraft from airline and military service, has resulted in reduced grade availability. Two grades are currently available in the United States. Fuel grades are designated by their anti-knock characteristics. Engine knock, which describes explosive detonation of the fuel/air mixture or preignition, can cause severe engine damage and subsequent failure in a short period of time. Anti-knock ratings are expressed as Octane Numbers for those of 100 or less and as Performance Numbers for those ratings above 100. These numbers relate the fuels performance compared to a reference fuel of pure isooctane. Because the anti-knock characteristics are influenced by the air/fuel mixture ratio, ratings are developed for both rich mixture performance and lean mixture performance. Rich mixture settings yield higher octane or performance numbers since the added fuel acts as an internal coolant and suppresses knock. Prior to 1975, both numbers were reported as the grade designation but current specification utilize only the lean mixture rating. Currently, ASTM (American Society for Testing and Materials) specifies three grades - 80, 100, and 100LL (low lead). In practice only 100LL is widely available. Grade 80 continues to be marketed but its distribution and availability is much more limited. Grade 100 is not now seen. With continuing modernization of the aircraft fleet over time, the demand for grade 80 continues to decline. It is expected that it will eventually reach a point when it is no longer economical to manufacture or use.