A concept described in StewartBrand's "HowBuildingsLearn" (chapter 2), based on the work of FrankDuffy.  Brand describes six "layers" common to buildings:

* Site - where the building is located
* Structure - the foundation and load bearing elements
* Skin - user interface
* Services - wiring, plumbing, heating, cooling, etc.
* Space Plan - interior layout
* Stuff - furnishings

He describes how each of these layers changes more rapidly than the last, and how buildings that don't allow that fail.  Buildings are often torn down because their antiquated services are embedded too deeply in their structure, for instance.

Avoid coupling things that change at different rates.  See also AlternateHardAndSoftLayers.