In early Welsh literature, an awdl was any long poem on a single end-rhyme (the word is the same as odl, 'rhyme'). Such early awdlau are associated with the Cynfeirdd such as Aneirin and Taliesin, and the poetry is found in manuscripts such as the Book of Taliesin, the Black Book of Carmarthen, the Hendregadredd Manuscript or The Red Book of Hergest.
Awdlau are, in the early period, to be distinguished from Englynion, which are short (three- or four-line) stanzas. Since the recorded beginnings, awdlau were highly ornamental, and the forms permitted became stricter and stricter until the high Middle Ages. The period of the 11th to 13th centuries saw the royal court poets (the Poets of the Princes) develop the art to a remarkable level of skill and accomplishment, and following the extinction of Welsh royalty with the death of Llywelyn ap Gruffudd in 1282, standardisation and codification of the rules of professional poetry led to the recognition of twenty-four strict metres, each of which must use cynghanedd. By this period, the englyn metres as well as the cywydd metres were included within the twenty-four, and the term awdl simpliciter became used for any long poem composed on metres chosen from the permitted range, with end-rhyme staying constant within individual sections of the poem. Among those known to have used the form in the early 17th century was Dafydd Llwyd Mathau in 1611.
Such poems are considered among the finest work that a poet can aim to produce, and prizes are given at eisteddfodau for the best awdl, including the most lauded prize, that of the Chair at the National Eisteddfod of Wales, held every year in August. While 19th-century awdlau could run to thousands of lines, it is unusual to come across such extensive compositions these days, and indeed, the National Eisteddfod limits competitors to a few hundred lines.
Probably the most famous awdl is the apocalyptic elegy composed by Gruffudd ab yr Ynad Coch on the occasion of the death in 1282 of Llywelyn ap Gruffudd (see above). A famous early twentieth-century example is  Yr Arwr, by Hedd Wyn.