What is periodos, and what does it mean to write in periods? The literal sense of the Greek word is ‘a going around’. It acquired technical senses from the Peripatetics, Aristotle and his followers, who used it to denote certain modes of rhythmically rounding out units of composition in music, in poetry, and in prose. In music, rhythm determines the values of tones and groupings with reference to any of a number of available measures. In poetry, rhythm arises from the interplay of words, phrases and sentences with and against an abstract metrical design. Prose, however, has no superordinate, sustained patterning comparable to the measures of music and verse. In prose, the compositional unit is the sentence; its rhythms result from particular dispositions of grammatically inflected words in phrases and clauses of varying length and complexity. Thus, when periodos was taken over into rhetorical theory and exposition, its domain could not be restricted to considerations of sound and rhythm, but necessarily had to include the effects of grammar and word choice, the forms of phrases and clauses, and the resulting conveyance of ‘thought’ or meaning. A basic notion of a rounding or a rounding-out did persist in the metaphorical extension of periodos as a technical term: one or more tactics for composing sentences that are at once comprehensive, goal-directed, and integrally complete, that is, ‘periodic’.