(See the discussion of iambic pentameter in the "Writing Style" section of our guide to the "General Prologue & Frame Story.")
An important aspect of the style of "The Reeve's Tale" is Chaucer's attempt to imitate the northern dialect of the clerks. This dialect replaces many of the o's in words with a's, and contains some words that would have been unfamiliar to someone who spoke the London dialect of English. So when the clerks first arrive at the mill, they say to John, "by God, nede has na peer. / Hym boes serve hymself that has na swayn" (172-173). In these sentences, o's have been replaced with a's, making the word "na" instead of "no." And instead of using the middle English word "moste" for "must," they've used the word "boes," a usage particular to the Northern dialect Chaucer is imitating here.