A hydrogen bond study was performed on dimethylammonium hydrobis(squarate), (H(2)NMe(2))(+)[H(HC4O4)(2)](-), using a detailed electron density distribution from accurate X-ray diffraction and ab initio molecular orbital calculations. The interaction energy between two H-bonded fragments and the electrostatic potential through the H-bond were also investigated. The anion of this compound contains both a symmetric (O...H...O distance of 2.4321(8) Angstrom, bond angle of 179(2)degrees) and an asymmetric (O-H...O distance of 2.5645(6) Angstrom, bond angle of 175(1)degrees) H-bond in the crystal. The compound crystallizes in the orthorhombic space group Peen. The cell parameters at 136(1) K are a = 15.614(3) Angstrom, b = 6.0537(7) Angstrom, c = 11.691(2) Angstrom, and Z = 4. Deformation density distributions are illustrated in terms of Delta rho(X-X), Delta rho(M-A), and Delta rho(theo); the former two are calculated from experimental data with the spherical and multipole atomic model, respectively, and the last one is derived from the ab initio calculation using a 3-21+G** basis set with the geometry obtained from the crystal structure. All these deformation density distributions give consistent agreement. The ring strain of the four-membered squarate ring is nicely demonstrated in the deformation density map with the bonding electron density maximum outward from the C-C interatomic axis (exocyclic), such that two neighboring bonding electron density maxima give an angle of 110 degrees around C2, which is much larger than the corresponding C1-C2-C3 angle of 90.83(4)degrees. Both the symmetric and asymmetric 11-bonds are illustrated in the deformation density maps; their differences are quite observable. The interaction energies of both types of H-bond are calculated; their values after basis set superposition error (BSSE) correction are -54.4 and -19.9 kcal/mol for symmetric and asymmetric, respectively. According to natural bond orbital (NBO) analysis, the charge transfer energy and/or the mutual polarization do play an important role in stabilizing the linear hydrogen-bonding system. A differential molecular electrostatic potential (DMEP) study demonstrates such charge transfer phenomena and gives a quantitative description about the potential well along the O...O line of the H-bonds.