Response: Spin-Singlet clusters in the ladder compound NaV2O5
  This response on a Comment on Reference 2 discusses the space group Fmm2 determination of the low temperature structure of NaV2O5. We show that while conventional crystallography is consistent with 'zig-zag' charge ordering of half of the ladders, recent spectroscopic measurements suggest that the symmetry is lower than Fmm2. We suggest space group C2, consistent with full 'zig-zag' charge ordering of all the ladders. 
J.L. de Boer, G. Maris, A. Meetsma, J. Baas, T.T.M. Palstra