(2) Linguistic usage may vary from one region to another; we have, then, to know the _language of the country_ where the document was written--that is, the peculiar meanings current in the country. (3) Each author has his own manner of writing; we have, then, to study the _language of the author_, the peculiar senses in which he used words.[136] This purpose is served by lexicons to a single author, as Meusel's _Lexicon Cæsarianum_, in which are brought together all the passages in which the author used each word. (4) An expression changes its meaning according to the passage in which it occurs; we must therefore interpret each word and sentence not as if it stood isolated, but with an eye to the general sense of the context. This is the _rule of context_,[137] a fundamental rule of interpretation. Its meaning is that, before making use of a phrase taken from a text, we must have read the text in its entirety; it prohibits the stuffing of a modern work with _quotations_--that is, shreds of phrases torn from passages without regard to the special sense given to them by the context.[138] These rules, if rigorously applied, would constitute an exact method of interpretation which would hardly leave any chance of error, but would require an enormous expenditure of time. What an immense amount of labour would be necessary if, in the case of _each_ word, we had to determine by a special operation its meaning in the language of the time, of the country, of the author, and in the context! Yet this is the labour demanded by a well-made translation: in the case of some ancient works of great literary value it has been submitted to; for the mass of historical documents we content ourselves, in practice, with an abridged method. All words are not equally subject to variations of meaning; most of them keep a fairly uniform meaning in all authors and in all periods. We may therefore be satisfied to study specially those expressions which, from their nature, are liable to take different meanings: first, ready-made expressions which, being fixed, do not follow the evolution of the words of which they are composed; secondly, and chiefly, words denoting things which are in their nature subject to evolution; classes of men (_miles_, _colonus_, _servus_); institutions (_conventus_, _justitia_, _judex_); usages (_alleu_, _bénéfice_, _élection_); feelings, common objects. In the case of all words of such classes it would be imprudent to assume a fixed meaning; it is an absolutely necessary precaution to ascertain what is the sense in which they are used in the text to be interpreted. "These studies of words," said Fustel de Coulanges, "have a great importance in historical science. A badly interpreted term may be the source of serious error."[139] And, in fact, simply by a methodical application of interpretative criticism to a hundred words or so, he succeeded in revolutionising the study of the Merovingian epoch.