Model children—the painter and his model—a model of a foundry—a miniature model of an aeroplane—Bohr-Rutherford's model of the atom. If one seeks the common element of these various objects and concepts, it would appear to be a comparison between two things displaying some common characteristics, one of which, at least, is the result of a creative effort on the part of man. One of these objects is called a model. But it can be the man-made one or the other which will be, for example, a given thing, either natural or artificial. A similar ambiguity exists in French with the word hôte; it is the notion of hospitality which prevails rather than the identification of one of the parties. In the case in question the relation is that of a form or structure which in both parties corresponds in some aspects, if not in all, to the supplementary criterion of human and voluntary origin of one of them. It also often happens that the thing called model in the relationship is single, whereas the other is multiple and varied both in its homologous characteristics and the additional ones it may possess. In the particular case of models used in science, the models themselves are a result of the creative effort of man.