  Binary supermassive black holes are produced by galactic mergers as the black
holes from the two galaxies fall to the center of the merged system and form a
bound pair. The two black holes will eventually coalesce in an enormous burst
of gravitational radiation. Here we show that the orientation of a black hole's
spin axis would change dramatically even in a minor merger, leading to a sudden
flip in the direction of any associated jet. We identify the winged or X-type
radio sources with galaxies in which this has occurred. The implied coalescence
rate is similar to the overall galaxy merger rate, suggesting that the
prospects are good for observing gravitational waves from coalescing
supermassive black holes.
