A lai (or lay lyrique, "lyric lay", to distinguish it from a lai breton) is a lyrical, narrative poem written in octosyllabic couplets that often deals with tales of adventure and romance. Lais were mainly composed in France and Germany, throughout the thirteenth and fourteenth centuries. The English term lay is a 13th-century loan from Old French lai. The origin of the French term itself is unclear; perhaps it is itself a loan from German Leich (reflected in archaic or dialectal English lake, "sport, play").The terms note, nota and notula (as used by Johannes de Grocheio) appear to have been synonyms for lai.

The poetic form of the lai usually has several stanzas, none of which have the same form. As a result, the accompanying music consists of sections which don't repeat. This distinguishes the lai from additional common types of musically important verse of the period (for example, the rondeau and the ballade). Towards the end of its development in the fourteenth century, a few lais repeat stanzas, but usually only in the longer examples. There is one quite late example of a lai, written to mourn the defeat of the French at the Battle of Agincourt (1415), (Lay de la guerre, by Pierre de Nesson) but no music for it survives.

There are four lais in the Roman de Fauvel, all of them anonymous. The lai reached its highest level of development as a musical and poetic form in the work of Guillaume de Machaut; 19 separate lais by this 14th-century ars nova composer survive, and they're among his most sophisticated and highly developed secular compositions.

Composers of lais