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).
The first Phantom IIs arrived on German soil in 1973 and were expected to fulfill their requirement for no more than ten years. July 2016: Turkey is pinning its strike fighter hopes on the Lockheed F-35 Lighting II series aircraft to replace its aging stock of McDonnell F-4 Phantom IIs in the same role. The F-35A will be fielded alongside the results of the indigenous TFX next generation fighter program currently in the works.