This paper sets out five constraints that a psychologically efficient orthography must meet. The first four principles are that (1) the elements of the orthography must be easy to discriminate; (2) it must be possible to write the code quickly, without elaborate technology; (3) the code must permit unambiguous, fast and fail-safe access to the meanings of messages; (4) the system as a whole should be learnable without undue expenditure of time and effort. A fifth principle, that the dimensions of formal and semantic similarity in an orthography should be orthogonal, is argued to be the best way of ensuring that reading errors are kept to a minimum. Evidence for the last principle is derived from the study of errors made by subjects with acquired dyslexia.