Abstract: A louver system for use with louver supports having inwardly opening perimetral end and side channel defining a rectangular louver space and adapted to receive the end and side edges of the louver. The louvers are provided with integrally molded end and side flanges, which may be of either of two preselected lengths, illustratively identified as short flanges and long flanges. The end flanges may have a length differing from the side flanges in the louver. Each of the short and long flanges may have different lengths depending on whether the cells of the louver are small or large. By providing the small cell louvers with either of the short or long flanges and the large cell louvers with either of the short or long flanges, a majority of standard louver mounting systems may be accommodated with only the four configurations.