On My First Son
We talk about the use (and reason) of rhyme in this poem over in "Form and Meter," so be sure to check that out for a full discussion. If we look a bit closer at the rhyme scheme, though, we notice that it's based on Jonson's use of assonance. In other words, it's the vowel sounds of the last words in each line that join each couplet together in rhyme.
For example, the long a sound in "pay" and "day" are what link lines 3 and 4. There are a couple of…couplets that might throw your modern ear, though. For example, check out lines 5 and 6. "Why" rhyme with "envy"? "Poppycock!" you might say. And we'd say, "Watch your mouth! Or else."
But then we'd say this: today's pronunciation is not the same as it was in Jonson's England a few hundred years ago, when "why" might have been pronounced "wee" and so would actually rhyme with "envy." The same might be said for lines 9 and 10. "Lie" would have been pronounced "lee," and so would have rhymed with "poetry." Do you see? Yessiree.