Slower layers can provide continuity while the faster ones allow for individuality. Designers need to provide solid foundation layers that can support faster changes on top of them, but they must also take care not to couple the layers too closely. An adaptive product “has to allow slippage between the differently-paced systems . . . Otherwise the slow systems block the flow of the quick ones, and the quick ones tear up the slow ones with their constant change.”34 Layer slippage, achieved through modularity, insures that a product remains cohesive even as it is changing.
Modularity can also help expose a product’s seams—visible clues about how the layers are joined and how they can be altered. In a building, seams might allow for easy access to wiring in ceilings or conduits, but what does it mean for products? One way to explicitly expose the seams of a product is through documentation. Home builder John Abrams has made a habit of thoroughly photographing the houses he builds before putting up the walls, so the owners can see where all the plumbing and wiring are. This photographic record helps make “later adjustment of the building so much easier. The photos reveal exactly where the Services go and what are the hidden Structural elements.”35 Imagine similar documentation for a product that shows where the layers are separated, how they it together, and how they can be changed. A product “needs a complete and accurate record of itself”36 so that people can avoid reverse engineering by trial and error when they want to make a change. Good documentation and visible seams enable maintenance of a product over a long period of time, and time is perhaps the most important requirement for successful evolution. Alexander acknowledges that adaptation happens slowly, that “you want to be able to