The repetition of sound (whether rhyme or alliteration) is part of what gives the language of Middle English poetry its musicality, the patterned artifice which signals that it is different from everyday speech. In rhyming verse, the rhyme at the end of each line (as well as the metrical pattern of the line) divides its language up into equal units. As well as marking the individual line, rhymes indicate greater divisions, whether these be pairs of lines linked together to form couplets (aabbcc), or larger patterns of quatrains (abab), tail-rhyme stanzas (aabaab), seven-line rhyme royal stanzas (ababbcc) or eight-line ballade stanzas (ababbcbc).
Rhyming verse in earlier Middle English often features what might seem to us to be less than ‘pure’ or ‘perfect’ rhyme such as assonance or half-rhyme. Some poets of the late fourteenth and fifteenth centuries (including Chaucer, Gower, Hoccleve and Lydgate) imposed stricter ‘rules’ on themselves, sticking mostly, but not always, to full rhyme. English verse borrowed from French poetry a type of rhyme called rime équivoque (that is, punning rhyme) in which the rhyme words are identical, whether these are rhyming homonyms (i.e. the wise and alle the wise, meaning ‘the wise ones’ and ‘in all respects’) or the same words used in two different senses or as two different parts of speech (i.e. the claw, the noun, and to claw, the verb). This is not poets failing to find a rhyme, but rather showing off their verbal ingenuity.
Be careful to take account of the demands that rhyme makes on a poet’s choice of words. Given the self-imposed constraints of getting a word which has the right rhyme in the right place in the line, poets manipulate language in various ways. If you are making a point about word order or syntax, think about whether the poet has inverted or altered normal syntax simply in order to get his rhyme in the right place. If you are making a point about verbal repetition or emphasis, consider whether the poet has repeated or added a word simply to pad out the line to push the rhyme-word into the right place.
Rhyme has an impact on language choice, too. Poets sometimes borrow a word into English from Latin or French to provide a rhyme, or use a rare word, or invent a nonce-word (i.e. a word coined in situ for a particular purpose, here rhyme). Chaucer occasionally makes use of variations in a word’s spelling or pronunciation in order to provide a rhyme word, or uses a variant from a different dialect (on this see Simon Horobin, Chaucer’s Language (2007), pp. 61–66). So if you want to make a point about a word in rhyme position, think about whether it has been chosen solely for its meaning, or more for its form and sound.
Middle English poets sometimes used verse fillers or tags (i.e. an often-used, largely meaningless phrase such as it is no faile, meaning ‘without a doubt’) at the end of the line to provide a rhyme (these can be checked via the Middle English Dictionary). The demands of rhyme may thus shape content in various ways. So if you are commenting on a poet’s choice of word, or interpreting a word which lies in rhyme position, consider whether the demands of form have influenced the content of the line. It isn’t persuasive to say that a stanza is really concerned with (for example) truth or doubt if those nouns only appear in tag phrases in rhyme position.
Rhyme highlights the similarity in sound and word-form in words which mean different things. In the most subtle and small-scale way, each rhyme asks whether the two (or more) words in question have any other relationship, whether this is similarity, difference, ironic contrast, or coincidence. It’s temptingly easy to pick out a pair or trio of rhymes in a commentary passage and argue for the profound significance of this pair or group. But every stanza in Troilus and Criseyde has rhymes, so it might be a bit superficial or arbitrary to pick out one set of rhymes, unless you have a really incisive interpretative point to make.
Some rhymes are used so often that they become familiar or expected. When a reader encounters one word, say a lover’s trouthe or ‘fidelity’, they might anticipate that a corresponding rhyme would refer to his lady’s routhe or ‘pity’. Poets can play with those expectations, side-stepping the expected rhyme or negating or questioning it. Myra Stokes and Owen Boynton, in the articles listed below, show how Chaucer exploits repeated and familiar rhymes in Troilus and Criseyde.
Marie Borroff, ‘Chaucer’s English Rhymes: The Roman, the Romaunt, and The Book of the Duchess’, in her Traditions and Renewals: Chaucer, the Gawain-Poet, and Beyond (2001), pp. 78–93