Double Bar. The double bar symbol is used to denote certain kinds of norms in mathematics (e.g., or). It is also used to denote parallel lines, as in, and in an older notation for the covariant derivative. In this work, the notation or is reserved for matrix and function norms, respectively.
The F-4 Phantom II remained in use by the U.S. in the reconnaissance and Wild Weasel (Suppression of Enemy Air Defenses) roles in the 1991 Gulf War, finally leaving service in 1996. It was also the only aircraft used by both U.S. flight demonstration teams: the USAF Thunderbirds (F-4E) and the US Navy Blue Angels (F-4J).
One MiG-21 was shot down on 21 February 1972 by a U.S. Air Force F-4 Phantom piloted by Major Lodge with 1st Lt Roger Locher as his WSO based at Udorn RTAFB, Thailand. This was claimed to be the first ever U.S. Air Force MiG kill at night, and the first in four years at that time.he MiG-21 was also used extensively in the Middle East conflicts of the 1960s, 1970s and 1980s by Egyptian Air Force, Syrian Air Force and Iraqi Air Force. The MiG-21 first encountered Israeli Mirage III Cs on 14 November 1964, but it was not until 14 July 1966 that the first MiG-21 was shot down.