Earlier aircraft designs generally featured canopies consisting of small plates of perspex (also known as Plexiglas) in a metal "greenhouse" framework, with the top of the canopy even with the rear fuselage; this was true of the IJNAS Mitsubishi A6M Zero, whose otherwise "all-around view" canopy was still heavily framed. This design considerably limited visibility, especially to the rear. The introduction of vacuum forming led to the creation of the "bubble canopy" which was largely self-supporting, and could be mounted over the cockpit, offering greatly improved all-round visibility. Tank's design for the Fw 190 used a canopy with a frame that ran around the perimeter, with only a short, centerline seam along the top, running rearward from the radio antenna fitting where the three-panel windscreen and forward edge of the canopy met, just in front of the pilot.