If you're already thinking that you'll name your next pair of pets Rhyme and Reason, we already have dibs on that. Okay, just wanted to get that out there.
Rhyme and Reason are the two beautiful princesses who have been exiled from the Kingdom of Wisdom and need to be rescued. Basically, they give Milo a reason to go on his quest. In their absence, the Kingdom of Wisdom is encountering all kinds of problems. When they were still around, Rhyme and Reason had tons of fans: "Everyone loved the princesses because of their great beauty, their gentle ways, and their ability to settle all controversies fairly and reasonably" (6.16). But without the princesses, no "controversies" can be "settle[d]." People don't respect sounds, or value words and numbers equally, and all the other things that Wisdom requires. When Rhyme and Reason are gone, so is rational thinking (hence their names.. got it?). In other words, without these two ladies, things in Dictionopolis and Digitopolis make no sense.
Okay, so if Rhyme and Reason are so smart and powerful, why can't they come back after they've been exiled? Well, when it comes down to it, their older brothers just have more authority than they do: the guys have their own cities and their own armies.
Whatever this can tell us about Rhyme and Reason as characters, it can tell us even more about what they represent. Norton Juster doesn't make it hard for us to figure out what Rhyme and Reason allegorize (represent): yep, that's right, rhyme and reason. So what does this mean for lowercase rhyme and reason? Maybe that power sometimes defies reason? Maybe that we should fight for reason, no matter how hard it seems? We can't be sure, but we do think it's worth thinking about.