The strong practical sense of Demosthenes refused to be confined by any theoretical rules of scholastic rhetoricians. He does not aspire to the complexity of periods which makes the style of Isocrates monotonous in spite of the writer’s wonderful ingenuity. Long and short, complex and simple sentences, are used in turn, and with no systematic order, so that we cannot call any one kind characteristic; the form of the sentence, like the language, is subordinate to its purpose.[352] He was moderately careful in the avoidance of hiatus between words, but in this matter he modified the rule of Isocrates to suit the requirements of speech; he was guided by ear, not by eye; thus we find that hiatus is frequently omitted between the _cola_ or sections of a period; in fact any pause in the utterance is enough to justify the non-elision of an open vowel before the pause. Isocrates, on the contrary, usually avoids even the appearance of hiatus in such cases. There is one other formal rule of composition which Demosthenes follows with some strictness; this is the avoidance of a succession of short syllables. It is notable that he very seldom admits a tribrach (three short syllables) where a little care can avoid it, while instances of more than three short vowels in succession are very exceptional.[353] An unusual order of words may often be explained by reference to this practice.[354] We know from Aristotle and other critics that earlier writers of artistic prose, from Thrasymachus onwards, had paid some attention to the metrical form of words and certain combinations of long and short syllables. Thrasymachus in particular studied the use of the _paeonius_ (–⏑⏑⏑ or ⏑⏑⏑–) at the beginning and end of a sentence.[355] The effect of increasing the number of short syllables, whether in verse or prose, is to make the movement of the line or period more rapid. The frequent use of tribrachs by Euripides constantly produces this impression, and an extreme case is the structure of the Galliambic metre, as seen, for instance, in the _Attis_ of Catullus.[356] Conversely the multiplication of long syllables makes the movement slow, and produces an effect of solemnity.[357] Demosthenes seems to have been the first prose-writer to pay attention to the avoidance of the tribrach; Plato seems to have consciously preferred a succession of short syllables where it was possible. The difference between the two points of view is probably this—that Plato aimed at reproducing the natural rapidity of conversation, Demosthenes aimed at a more solemn and dignified style appropriate to impressive utterance before a large assembly.