 A little history will show how the word ‘model’ came to have these two different uses. In late Latin a ‘modellus’ was a measuring device, for example to measure water or milk. By the vagaries of language, the word generated three different words in English: mould, module, model. Often a device that measures out a quantity of a substance also imposes a form on the substance. We see this with a cheese mould, and also with the metal letters (called ‘moduli’ in the early 17th century) that carry ink to paper in printing. So ‘model’ comes to mean an object in hand that expresses the design of some other objects in the world: the artist’s model carries the form that the artist depicts, and Christopher Wren’s ‘module’ of St Paul’s Cathedral serves to guide the builders.