A simplified sketch of the setup used by Weymouth et al. . Pairs of hydrogen atoms (bonded to silicon atoms below them) form “dimers” (represented by copper-colored ovals) that are aligned within a single atomic terrace. Because silicon surfaces are cut at slight angles, the flat atomic terraces are separated by surface steps. Resulting from the crystal structure, the dimers on alternating terraces are aligned at 90° with respect to neighboring terraces. A tungsten tip coated in silicon atoms (obtained by first briefly touching the tungsten tip to the surface) is brought towards the surface to within approximately one atomic distance. The tip, directly connected to a quartz oscillator that serves as a force sensor, is oscillated parallel to the surface, aligned with or perpendicular to the direction of one of the dimer rows. The forces acting between the tip and the aligned dimer rows vary as the tip traverses the step between terraces, resulting in a variation in the measured frequency shift of the force sensor.