For a discussion of Chaucer's use of iambic pentameter check out our guide to the "General Prologue & Frame Story." Here we'll discuss the style unique to "The Second Nun's Tale."
The rime royal stanza is a poetic form that Chaucer is actually responsible for originating in English. It's a seven-line stanza with the line endings forming an ABABBCC rhyme scheme. In The Canterbury Tales, Chaucer uses this form for "The Clerk's Tale," "The Man of Law's Tale," "The Second Nun's Tale," and "The Prioress's Tale." Since these are all religious tales about pious figures, it seems that Chaucer used the rime royal stanza to convey a serious moral purpose.
The rime royal stanza is a slightly more difficult one for Chaucer to pull off than just the simple rhyming couplets he usually prefers, because for each B-rhyme he writes, he has to come up with not just one other rhyme, but two. Take this passage, for example:
First wolde I yow the name of seinte Cecilie A
Expowne, as men may in hir storie see. B
It is to seye in Englissh, `hevenes lilie' A
For pure chaastnesse of virginitee, B
Or for she whitnesse hadde of honestee B
And grene of conscience, and of good fame C
The soote savour, lilie was hir name. C
Here, Chaucer has to come up with two rhymes for "see," which he does with "virginitee" and "honestee." This pressure to produce two end rhymes often causes Chaucer to invert the normal English order of sentences, so that instead of saying "First I will explain Cecilia's name to you" he writes "First will I Cecilia's name explain to you." Doing this gives him more flexibility with end rhymes, but it also makes his poetry more difficult to read.