Highly purified silicon single crystals are excellent standards for length measurements at the atomic scale and the value of the silicon (220) lattice spacing is an important item of information for the establishment of a self-consistent set of fundamental physical constants. This article surveys silicon d(220) determinations by describing the measurement method and the most important features of x-ray/optical interferometry. A summary of the d(200) values so far obtained is also given. The measurement procedure is described in detail, with emphasis on the critical points deserving particular attention in error analysis. Bottle-necks evidenced in experiments are pointed out and the possible evolution of x-ray/optical interferometry is analysed.