The words of the international language consist of invariable elements (morphemes) of three sorts: stems, derivative affixes (prefixes and suffixes), and grammatical inflections which, as in the case of European languages, are always final letters or final syllables. The stems themselves can be divided into two categories: verb stems, which express a state, action, or relation, _e.g._, _dorm_, _parol_, _frap_; and non-verbal or nominal stems, which denote an object (living being or thing), or express an aspect of it, _e.g._, _hom_, _dom_, _bel_, _blind_. The latter can produce directly only names (substantives or adjectives): man, house, beautiful, blind (in Ido, _homo_, _domo_, _bela_, _blinda_); the former, on the contrary, produce directly verbs: to sleep, to speak, to strike (in Ido, _dormar_, _parolar_, _frapar_), but they can also give rise to nouns: sleep, word, blow (in Ido, _dormo_, _parolo_, _frapo_). The proper _rôle_ of the grammatical terminations is to determine the grammatical function of a stem word and to indicate the category to which the word belongs, whether verb, substantive, or adverb. Thus _parol-ar_ = to speak; _parol-o_ = (spoken) word; _parol-a_ = oral; _parol-e_ = orally. The _same_ idea, namely, that expressed by the stem word, always runs through the various categories. This follows from a principle which dominates the whole structure of the international language: "Every word element" (morpheme) "represents an elementary idea, which is always the same, so that a combination of elements has a meaning determined by the combination of the corresponding ideas." This principle is only a corollary to the general principle of uniqueness so clearly enunciated by Ostwald: "There exists a unique and reciprocal correspondence between the ideas and the morphemes which express them." This principle represents evidently the ideal of all language, for a language, being essentially a system of symbols, is only theoretically perfect (and useful and convenient in practice) when there exists a unique correspondence between the symbol and the idea symbolised.