Early canopies were made of numerous pieces of flat glass held in position by a frame and muntins. The muntins reduced visibility, which was especially problematic for military aircraft. Also, glass canopies were much heavier than acrylic canopies, which were first introduced shortly before World War II. The acrylic bubble canopy was used on aircraft such as the Supermarine Spitfire and Westland Whirlwind, which gave better all-round visibility and reduced weight. It is still being used today on most fighter aircraft.