Analytical models are a fundamental tool in the development of computer-based systems of every kind: their essential purpose is to support human understanding and reasoning in development. To support reasoning, models must be substantially formal. The relationship between a formal model and its--typically--non-formal subject demands care: particular attention must be paid to the model interpretation, which maps its formal terms to the phenomena of the subject. An analytical model is to be regarded not as an assertion, but as a predicate within a larger logical structure of reasoning. Analogical models, such as databases, act as run-time surrogates for some parts of the problem world; in their design the properties of the model itself must be carefully distinguished from those of its subject. Some models may be informal: informal models have many legitimate uses, but cannot serve as a basis for formal reasoning. 1 Modelling and Understanding The subject of these notes is the use of mode...