As a combination example, Figure 4 shows a coordinate system B that orbits inertial frame A as in Figure 3, but the coordinate axes in frame B turn so unit vector u 1 always points toward the centre of rotation. This example might apply to a test tube in a centrifuge, where vector u 1 points along the axis of the tube toward its opening at its top. It also resembles the Earth–Moon system, where the Moon always presents the same face to the Earth. In this example, unit vector u 3 retains a fixed orientation, while vectors u 1, u 2 rotate at the same rate as the origin of coordinates. That is,