A kyrielle is written in rhyming couplets or quatrains. It uses the phrase "Lord have mercy", or a variant on it, as a refrain as the second line of the couplet or last line of the quatrain. In less strict usage, other phrases like "O God, be merciful to me", and sometimes single words, are used as the refrain.
A kyrielle is a poem type that is made of quatrains. Quatrains are four stanza poems. A kyrielle is actually a French form of poetry. The kyrielle is also a rhyming poem so the quatrains must, in fact, rhyme in some way. Each line of a kyrielle poem must consist of eight syllables. The rhymes in a kyrielle occur in couplets, meaning every two lines rhyme with one another. A kyrielle poem can be as short or as long as the writer wishes, so long as it contains the original rhyming quatrain, it is a kyrielle.