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Rat trap bond

This brick laying pattern is known as rat trap bond. All the bricks are laid on their sides instead of flat. The headers (the ones with the smallest end showing) are laid across the wall and form a similar pattern the other sides. But that means that the stretchers (the long sides showing) effectively form two parallel walls with a gap between them. It is this gap (about 2 inches) that forms the rat trap. Walls that did not need to be structural were built like this because they use use fewer bricks and are therefore cheaper.