Abstract: A corner clip apparatus and method is provided for closing open corners formed by duct flanges. The corner clip can be shipped flat and bent into the useable form at the worksite using hand tools, or pre-bent during manufacture. The corner clip comprises a planar body with first and second limbs, first and second outer flap tabs for folding below the body and engaging with duct flanges adjacent the open corner to be closed, and first and second sets of inner restraining tabs for bending over and engaging with the inner duct wall to laterally secure the corner clip to the duct. A hole can be drilled through the first and second limbs during manufacture to serve as pilot holes to drill through corresponding duct flanges and flap tabs once the corner clip is installed to allow for rivets to be installed to further secure the corner clip to the flanges.