* The 2×4 rule describes the distance between elements making up a pattern applied to windows for the purpose of preventing bird strikes. To be effective, the pattern must uniformly cover the entire window and consist of elements of any shape (lines, dots, other geometric figures, etc.) separated by no more than 5 cm (2 inches) if oriented in horizontal rows, or by 10 cm (4 inches) if oriented in vertical columns. These patterns eliminate bird-window collisions when applied to the outer surface (Surface #1) of reflective panes, or when applied to any surface of multi-pane see-through windows. Greater spacing between pattern elements increases the risk of a strike and casualties.