In a general coordinate system, the basis vectors for the coordinates may vary in time at fixed positions, or they may vary with position at fixed times, or both. It may be noted that coordinate systems attached to both inertial frames and non-inertial frames can have basis vectors that vary in time, space, or both. For example, the description of a trajectory in polar coordinates as seen from an inertial frame or as seen from a rotating frame. A time-dependent description of observations does not change the frame of reference in which the observations are made and recorded.