These were effectively prohibited in the US, where standard-size sealed beam headlamps were mandatory and intensity regulations were low. US lawmakers faced pressure to act, due both to lighting effectiveness and to vehicle aerodynamics/fuel savings. High-beam peak intensity, capped at 140,000 candela per side of the car in Europe, was limited in the United States to 37,500 candela on each side of the car until 1978, when the limit was raised to 75,000. An increase in high-beam intensity to take advantage of the higher allowance could not be achieved without a move to halogen technology, and so sealed-beam headlamps with internal halogen burners became available for use on 1979 models in the United States. High-intensity discharge (HID) systems appeared in the early 1990s, first in the BMW 7 Series. 1996's Lincoln Mark VIII was an early American effort at HIDs, and was the only car with DC HIDs. Beyond the engineering, performance and regulatory-compliance aspects of headlamps, there is the consideration of the various ways they are designed and arranged on a motor vehicle. Headlamps were round for many years, because that is the native shape of a parabolic reflector. Using principles of reflection, the simple symmetric round reflective surface projects light and helps focus the beam. There was no requirement in Europe for headlamps of standardized size or shape, and lamps could be designed in any shape and size, as long as the lamps met the engineering and performance requirements contained in the applicable European safety standards. Rectangular headlamps were first used in 1961, developed by Cibié for the Citroën Ami 6 and by Hella for the German Ford Taunus. They were prohibited in the United States where round lamps were required until 1975. Another early headlamp styling concept involved conventional round lamps faired into the car's bodywork with aerodynamic glass covers, such as those on the 1961 Jaguar E-Type, and on pre-1967 VW Beetles. Headlight design in the U.S. changed very little from 1940 to 1983. In 1940, a consortium of state motor vehicle administrators standardized upon a system of two round sealed beam headlamps on all vehicles—the only system allowed for 17 years. However, the Tucker 48 included a defining "cyclops-eye" feature: a third center-mounted headlight connected to the car's steering mechanism. It only illuminated if the steering was moved more than ten degrees off center and the high beams were turned on.