This paper is a practical pocket philosophy of building construction. It sets out a way of thinking about building construction in terms of principles, rules and compensation, and illustrates their application with a range of examples. Its basis lies in reflections on the way things work and the way they fail, and a search for a simple pattern in this. Its aim is to set out a system which by virtue of its simplicity will serve as a guide both to those designing and to those interpreting buildings. The ideas it contains were first set out by Lyall Addleson and the author in Performance of Materials in Buildings (Butterworth Heinemann, Oxford, 1991). The focus of these ideas is those parts of a building which act as containers, separators, and modifiers of environments or, in other words, the construction of the building envelope.