The theory of Oral-Formulaic Composition is the mechanism proposed for how the Homeric epics could have been passed down through many generations purely through word of mouth. While most modern scholars believe that the Iliad and Odyssey were preserved through large periods of time through an oral tradition (a series of storytellers who would re-tell the stories and pass them on to their children for generations), the simple mechanics of how Greek bards could remember such long epics are hard to understand. According the theory, multiple generations of bards who would recite epics such as the Iliad and Odyssey to aristocrats developed a series of verbal formulas to aid their memories. This resulted in the many epithets found in Homeric verse, such as "the red-haired king" for King Menelaus; however, it also gave rise to the many longer depictions of generic actions in epic, such as the steps taken to arm oneself or prepare a ship for sea. Because of these many formulas, bards who had a general outline of the story they needed to tell could essentially improvise individual passages that have more to do with details.
In Homeric verse, for example, a phrase like eos rhododaktylos ("rosy fingered dawn") or oinops pontos ("winedark sea" occupy a certain metrical pattern that fits, in modular fashion, into the six-colon Greek hexameter, and aid the aioidos or bard in extempore composition. Moreover, phrases of this type would be subject to internal substitutions and adaptations, permitting flexibility in response to narrative and grammatical needs: podas okus axilleus ("swift footed Achilles") is metrically equivalent to koruthaiolos ektor ("glancing-helmed Hektor"). Parry and Lord observed that the same phenomenon was apparent in the Old English alliterative line:
and in the junacki deseterac (heroic decasyllable) the demonstrably oral poetry of the Serbs:
Though the Oral-Formulaic theory is often discussed in conjunction with Homeric Epic, it can be applied to ancient epic poetry worldwide. In fact, it was first proposed by Milman Parry and his student Albert Lord as a way to explain how illiterate Serbian and Croatian singers could recite long narratives.