These images contain top and side views of the wake produced by a rigid plate pitching about its leading edge in a uniform flow (flowing left to right). The leading edge of the plate is hinged to the trailing edge of a stationary symmetric airfoil. The wake is visualized using fluorescent dyes that are introduced through a series of holes on each side of the airfoil support. Twice in each flapping cycle, a horseshoe-shaped vortex is shed from the top, bottom, and trailing edges. The vortices become entangled to form the chain-like structure shown here. Studying such wakes is believed to be important for understanding the mechanisms of thrust production in fish-like swimming.