How were poems composed and passed down through generations in Anglo-Saxon Britain?

Oral-traditional lterature is not handed down verbatim
over the generations but rather composed as it is performed from traditional elements.
The traditional elements used in oral-forulaic composition operate on the levels of
story, scene and line. A "singer of tales" learns the overall narratives of traditional
tales by listening to other performers. He then accumulates knowledge of standard
patterns of scnes, e.g. the "hero donning armor" or "hero leaving home" scenes. To shape
peotic lines in performance, the singer learns standard formula -- normal epithet-noun
and verb phrases that fit metrically into partial or complete lines and then assembles
lines out of these components. No two performances of the "same" oral-traditional epic
are identical by the identity standards of literate societies, but instead "the same
tale" means the same general narrative elaborated in response to performance situation
by the singer, just as the "same melody" may be arranged differently by different
musical ensembles.