Museo Alu-Frame Aluminum Stretcher Bars feature a reinforced structure comprised of wear-resistant aluminum and sturdy wood for maximum stability. Unlike all-wood stretcher bars, aluminum bars are unaffected by changes in humidity and temperature. They won't twist, bend, or warp. - MUSEO ALU-Frame Aluminum Stretcher Bars and Parts - 4" Extender, 1-3/4" Profile, Pkg of 2Museo Aluminum Stretch Bar, SET 6 - Attach two stretcher bar lengths together using an extender to make a frame up to 204" long. Extenders are available for both profile sizes and sold in packs of two.