Colored airways are all depicted in brown on low and high altitude charts produced by the FAA's Aeronautical Navigation Products. Low altitude airways (below MSL) that are based on VOR stations, appear on sectional charts, world aeronautical charts, and en route low altitude charts and are designated with the prefix "V" (pronounced "victor", hence, "victor airways"). High altitude airways (from MSL to FL450) based on VOR stations are called jet routes; they appear on high altitude charts (that usually don't show topography, as the low altitude charts do) and are prefixed by the letter "J". VOR-based routes are depicted in black on low and high altitude charts produced by the FAA's Aeronautical Navigation Products. With the invention of RNAV routes, airway structure no longer has to be based on ground-based navaids; a new naming convention is used. RNAV routes not based on VOR routes in low altitudes are preceded with the letter "T"; high airway routes are designated with the letter "Q". RNAV routes are depicted in blue on low and high altitude charts produced by the FAA's Aeronautical Navigation Products. Airways are corridors wide of controlled airspace with a defined lower base, usually FL070–FL100, extending to FL195. They link major airports giving protection to IFR flights during the climb and descent phases, and often for non-jet aircraft, cruise phase of flight. Historically, they were laid out between VORs; however, advances in navigational technology mean that this is no longer a necessity. Each airway has a designator containing one letter and one to three numbers. All airspace above FL195 is class C controlled airspace, the equivalent to airways being called Upper Air Routes and having designators prefixed with the letter "U". If an upper air route follows the same track as an airway, its designator is the letter "U" prefix and the designator of the underlying airway. In the UK, airways are all class A below FL195 and therefore VFR flights are prohibited. An air corridor is a designated region of airspace that an aircraft must remain in during its transit through a given region. Air corridors are typically imposed by military or diplomatic requirements. During the Berlin Blockade, for example, pilots flying across Soviet-controlled German airspace were required to maintain very specific positioning within air corridors defined by the commander in charge of the airlift.