A situation in which the axis of rotation is physically constrained to be in a set location relative to the moving parts, usually because it is fabricated as an axle. This is not invariably the case – sometimes the center of rotation is fixed even though there is no physical axis in place, but the result is the same – the system is forced to rotate about one well-defined axis. Unlike, for example, a top, whose axis can change direction, or a coin flipped into the air, where the axis of rotation is teslf moving through space.