Regular domain boundaries (RDBs) on a Si(111)-7×7 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)-7×7 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. © 2000 American Vacuum Society.