The 3,000 double-hung, 12-over-12 pane windows formed a critical visual element in the rehabilitation of the Boott Cotton Mills in connection with the development of Lowell National Historical Park. Most original wood frames could be repaired and retained, but sash required replacement. This note describes the design and fabrication of aluminum double-glazed sash replicating the visual effect of the original windows, and evaluates the results. <i>Windows</i>, Number 18 in a series.