The atomic geometry of Si(001) with a misorientation of 4.5° towards  has been studied with scanning tunneling microscopy. Special attention has been paid to the buckling registry of the dimers and atoms near the biatomic step edge. For the arrangement of the atoms near the steps and at kink positions some simple rules are given. The biatomic step edge formation energies have been determined by analyzing the step edge roughness. The ordering of the various types of step edges is in perfect agreement with total energy calculations performed by Chadi [Phys. Rev. Lett. 59, 1691 (1987)]. Finally, the transition from a surface with single steps to a surface with double steps has been calculated to occur at a miscut angle of 1.5°.