The controlled assembly of nanowires is a key challenge in the development of a range of bottom-up devices(1,2). Recent advances(2-19) in the post-growth assembly of nanowires and carbon nanotubes have led to alignment ratios of 80-95% for a misalignment angle of +/- 5 degrees (refs 5,12-14) and allowed various multiwire devices to be fabricated(6,10-13,19). However, these methods still create a significant number of crossing defects, which restricts the development of device arrays and circuits based on single nanowires/nanotubes. Here, we show that a nanocombing assembly technique, in which nanowires are anchored to defined areas of a surface and then drawn out over chemically distinct regions of the surface, can yield arrays with greater than 98.5% of the nanowires aligned to within +/- 1 degrees of the combing direction. The arrays have a crossing defect density of similar to 0.04 nanowires per mm and efficient end registration at the anchoring/combing interface. With this technique, arrays of single-nanowire devices are tiled over chips and shown to have reproducible electronic properties. We also show that nanocombing can be used for laterally deterministic assembly, to align ultralong (millimetre-scale) nanowires to within +/- 1 degrees and to assemble suspended and crossed nanowire arrays.