The O-ring was a part of the Shuttle's solid rocket booster that, when working correctly, stopped hot gas escaping through joins in the booster's steel skin called field joints. Each booster had three field joints, and each joint had two rubber O-rings, a main and a backup, that circled the booster. When the boosters were tested in 1977, however, the force caused by the pressure inside the booster made the steel parts bend away from each other at the field joints, breaking the O-ring seals and allowing the hot gases to escape. This was called "joint rotation", and if the "blow-by" of the hot gases against the O-rings was bad enough it could have destroyed the ring and caused the booster to explode. However, it was also found that the rubber O-rings would shift under the forces of lift-off and form a seal anyway, which was called "extrusion", so the problem was not seen as serious by NASA.