A class of structural models is studied for the Si(l 1 l)-(7x7) surface which is consistent with scanning tunneling microscopy (STM) results. The hills observed with STM are identified with trimers that are formed by bond flipping. The valleys correspond to boundaries between differently stacked triangular areas. Atoms with broken bonds are removed along these boundaries. A modified Keating strain calculation is used to determine the orientation of the trimers and to obtain accurate atomic positions. The trimer models exhibit large atomic displacements perpendicular to the surface accompanied with small parallel displacements in qualitative agreement with Rutherford backscattering results. The structural changes upon hydrogenation of Si(111)-(7 X 7) are explained by a reversal of the bond flip whereby the stacking is not altered. © 1984, American Vacuum Society. All rights reserved.