Stewart Brand’s revised diagram of time-based building systems, based on Frank Duffy’s categories, but with two more S’s and some changed names (“site, structure, skin, services, space plan, and stuff”), each with its own characteristic time-frame for repair, maintenance, or replacement (image by J. Ochshorn adapted from an image by Stewart Brand, How Buildings Learn, p.13, which was, in turn, adapted from an image by Frank Duffy)