Medieval schoolbooks normally dealt fairly briefly with the subject of poetic diction. Their standard procedure was to list the rhetorical figures and tropes (figurae et tropi) of Latin literature and to exemplify them briefly with citations from major poets, both classical and, after Bede, Christian. Figures and tropes were conventionally regarded by medieval grammarians as ornaments of poetic diction rather than integral features, an essentially superficial view that did not really suit a discussion of the nature of skaldic poetry, where kennings and heiti were fundamental to the practice of the skalds and formed a distinctive poetic metalanguage. Another conventional medieval attitude to poetic language, which we see influencing both Snorri's work and the Third and Fourth Grammatical Treatises, was to evaluate figures and tropes in terms of a classical sense of decorum and then to classify them as stylistic virtues or vices, depending on whether they ornamented the poem harmoniously in accordance with what was regarded as natural or not. Fundamental, also, was an understanding of the nature of the process by which figurative language operates. Figurative language caused a problem for medieval poetic theory, as it was seen as in a sense ‘unnatural’. The standard explanation, which was to be found in all elementary grammatical treatises, was that every word had its own ‘proper’ (proprius) meaning, which is what we would call the ‘literal’ sense of a word. This was conceived of as singular, that is, there was thought to be a one-to-one relationship between a word and its meaning, whereas, as modern semantics tells us, many words in human languages have several senses in common usage. When a word is used in a metaphorical or figurative sense it clearly assumes a different meaning from its literal counterpart, though the two are related; the standard medieval explanation for this new meaning was that the word had taken on a meaning that was not really its own, that is, it had assumed for the moment an ‘improper’ (improprius) sense.

Óláfr þórõarson translates what the late classical grammarian Donatus has to say on the subject of the trope with its transferred meaning from a ‘proper’ to an ‘improper’ sense in chapter 16, de tropo et metaphoræ, of his Third Grammatical Treatise: