  In this paper, a potential field extrapolation and three nonlinear force-free
(NLFF) field extrapolations (optimization, direct boundary integral (DBIE) and
approximate vertical integration (AVI) methods) are used to study the spatial
configuration of magnetic field in the quiet Sun. It is found that the strength
differences between the three NLFF and potential fields exist in the low
layers. However, they tend to disappear as the height increases, which are of
the order of 0.1 G when the height exceeds $\sim$ 2000 km above the
photosphere. The absolute azimuth difference between one NLFF field and the
potential field is as follows: for the optimization field, it decreases
evidently as the height increases; for the DBIE field, it almost keeps constant
and shows no significant change as the height increases; for the AVI field, it
increases slowly as the height increases. The analysis shows that the
reconstructed NLFF fields deviate significantly from the potential field in the
quiet Sun.
