Regular domain boundaries (RDBs) on a Si(111)-7 x 7 surface are investigated both theoretically and experimentally. Based on the assumption that the RDBs form from a relative shift between two adjacent parts of the perfectly reconstructed lattice, a set of indexes is proposed to classify, the RDBs. The irreducible indexes are obtained according to the constraints derived from the lattice-matching condition and the symmetry of the Si(111)-7 x 7 structure. Along the RDBs, dimer rows should be constructed serving as backbones, and the defects in the RDB should be accompanied by a dimer-adatom stacking fault subunit or unit spanning the RDB as a matching bridge. Most of the RDBs we obtained by direct annealing of the: Si(111) surface and through solid phase growth have such features. The atomic structural models compared to the STM images of several examples are presented,