About 13 years ago rumours began to emerge from the USA of a hitherto unknown and mysterious condition afflicting the F100 and other contemporary fighter aircraft. In fast rolls these aircraft were said to experience gyrations in yaw and in pitch which by all accounts had no right to exist, especially as aerodynamically these configurations could not be faulted. After a burst of theoretical and experimental activity the condition was explained and the terms inertia cross-coupling, roll-coupling, roll yaw coupling and other synonyms became common currency in the world of aeronautics. The mystery of these puzzling manoeuvres was essentially solved. Unfortunately the problem was not. Cross-coupling has since become a major item on the agenda of the designer of every highspeed fighting aircraft and more recently it has also made some incursions in the area of the large slender aircraft.