In oral epic, the basic building block is the formula: a group of words regularly employed under the same metrical conditions to express a given essential idea. The singer does not memorize fixed lines but composes in performance by drawing on a learned stock of such patterns, which lets him produce metrically correct verse at speed.
Beyond single lines, singers build narrative from themes: recurrent scenes such as arming, feasting, assembly, or journey that exist in multiple variant realizations rather than one fixed wording. The singer selects and expands or contracts these thematic clusters to fit the occasion, so the same story can be told at different lengths without the singer feeling he has altered it.
Each performance of an oral epic is a fresh composition, not a recitation of a memorized original, so there is no single authoritative text behind the variants. Asking for the 'correct' version misunderstands the tradition, because the song exists only as the sum of its performances.
Ancient and medieval texts can be examined for the density of formulas, thematic repetition, and ring composition to assess whether they emerged from an oral tradition rather than literate authorship. This treats stylistic features not as ornament but as diagnostic evidence of a work's mode of composition and transmission.
In 1930s Yugoslavia, Milman Parry and Albert Lord recorded illiterate South Slavic singers (guslars) performing heroic epics to the gusle, and by having the same singer perform a song on different occasions documented how wording shifted while the story held. These field recordings supplied the living laboratory from which the formula-and-theme model of oral composition was derived and then applied back to Homer.