Continued from part 1, where we learn that I am a poet and I know it.
What I must stress
Following on from what I've said (well, written), it should be clear that the first, stressed syllable of a rhyme is the most important. If two paired stress positions bear an identical vowel, that is the first clue (or confirmation) that a rhyme has begun. From the "poet"/"know it" example, however, we know that we can "get away with" slight differences in a following unstressed vowel and, to a lesser extent, intermediate consonants. The terminal consonant is like the flourish at the end of a gymnastic tumble: best to leave your audience remembering the grace of your finish, however ungainly some of the journey may have been; leave your audience in no doubt that what you just said (or wrote) was purporting to be a rhyme. The middle bit should be as cleanly executed as you can get it, but there may be room for "artistic expression". Of course, if your stressed syllable is the last in the line, you have only one vowel and the terminal consonants to work with, so there is no middle bit, just a standing somersault.
How to squeeze a vowel
Do you have trouble spelling long words? How many times have you seen mis-spellings such as "definat[e]" or "seperate" compared to the generally correct spellings of "definition" and "apart"? (That last question, at least, was rhetorical: I will not deduct marks if you leave it blank.) The point is that unstressed vowels tend to degrade into an indistinct schwa (a short burst of a "vowel" where the underlying pitch delivered is actually a broad range of frequencies), leading us not to hear what is written. In the case of "poet"/"know it", the second vowel is not quite a true schwa, but it is well on the way: the short 'e' of "poetic" is nowhere to be heard. It is perhaps better to think of there being a family of schwas and near-schwas. (In fact, this level of subtlety is explicitly acknowledged in written Hebrew by the "pointing" that is sometimes placed under a consonant to indicate the quality of the following vowel: a symbol resembling a colon indicates a totally indistinct vowel; a pseudo-colon alongside another vowel symbol indicates a degraded version of that vowel.) As a crude caricature, in American English indistinct vowels are more likely to approximate an "uh" sound, while in British English there are more semi-distinct short 'i' sounds; however, most of us have heard enough accents (and solecisms) to be cajoled into squeezing an unstressed vowel into a schwa (or quasi-schwa) without difficulty. And if we are aiming for comic effect, squeezing a longer vowel into a schwa, or even stretching a schwa or quasi-schwa to match a longer vowel, can be a whimsical bonus. If your vowels combine in a diphthong, however, it is very difficult to stretch or squeeze your way towards an acceptable rhyme; in practice, most diphthongs will grab some level of secondary stress wherever they occur in a word. And if there is secondary stress, best stick to a perfect match.
How to corrupt a consonant
Consonants are, broadly speaking (and here you may appreciate the limits of my formal linguistics training), the punctuating sounds between, or either side of, vowels. They are mainly percussive, although they can be a soft transition ("nowell"), aspirate ("ahoy") or a glottal stop ("a nice cu' o' tea"), amongst other possible sounds. Many consonants come in voiced and unvoiced flavours (e.g. b/p, g/k, d/t); an unvoiced consonant can often be easily corrupted and lightly voiced for a satisfactory rhyme: "Climb the ladder. / What's the matter [madder]?". There are also some superficial similarities between consonant sounds which are produced very differently, most notably 'f' and an unvoiced 'th' ("fings ain't what they used to be"), meaning that you (or I) can - at a push - rhyme "encephalopathy" with "proper fee". This last example also demonstrates the southern British and Bostonian trait of not vocalising terminal 'r's, which involves more corruption to achieve in some other accents. Without taking too many liberties, it is possible to rhyme duplicate with single consonants ("swooping" / "soup ping") and/or ignore aspirates ("shopper" / "stop her"). But remember to try and be not too corrupt when starting any syllables which bear a secondary stress - differences are more likely to be noticed.
Consonants can be sociable and often cluster in consonant blends. In the middle of a rhyme, it generally doesn't matter where your word division is, as long as all the consonant sounds are represented ("clock stop" / "box top"). If you have a large cluster of consonants, the listener will generally be more inclined to overlook minor corruption in one sound.
There are a limited number of consonant blends which will naturally start a syllable. It may be the most natural thing in the world to say "cops" and "baps", but almost nobody pronounces the 'p' in "psychology" (which is silent, as in water, according to the classic joke). This can help you to determine where your syllable begins: keep crediting consonants to the following vowel until the blend is unnatural to say if you pretend it is the start of a new word. Conventionally, syllables start with a consonant or consonant blend, if possible, and end, wherever possible, on an open vowel, closed only by those consonants that cannot be pronounced as part of an initial consonant blend.
Now, why does this matter?
Homophones and pseudo-homophones
In French, where rhymes are usually on one syllable and word stresses are not very insistent, homophones are greatly prized. In English, they often sound too similar. In the past, I have rhymed "peer" with "appear". Remembering that syllables start with a consonant where possible, both rhymed portions sound identical. It was only on reading the offending poem aloud that I heard the jarring sound of pure repetition, which reads to the mind as laziness, as if I had just repeated the same word because I couldn't think of a rhyme. Perhaps the greatest serial offender here is the pairing of "leave" and "believe".
But what if I paired "leave" (or "believe") with "sleeve"? No such problem. The 'sl' consonant blend is a natural start to a syllable, so the syllables sound different. But that does not mean I can rhyme "believe" with "this leave"; most people will hear the word division and have the syllable start with 'l' rather than 'sl'. The conventional concept of a syllable is sometimes trumped by knowledge of stand-alone words. Conversely, although I will never rhyme "all" with "at all", the latter idiom behaves so much like a single word that it would sound far worse to rhyme it with "tall"; the word "another", however, has crossed the threshold and rhymes relatively comfortably with "other" for most people.
Now it gets complicated. Consider which of the following words will rhyme euphoniously: "press", "express", "compress". Ultimately, euphony is in the ear of the beholder. To my ear, for what it's worth, I find it difficult to suppress (see what I did there?) the knowledge that "compress" and "express" (rendered "ek-SPRESS") contain the same root word meaning much the same thing. Although the final syllables are not strictly homophones, my brain registers a rhyme cop-out similar to if the word "press" on its own had been repeated. Others may disagree, but it would be much safer to find an alternative, etymologically unrelated rhyme, rather than appealing to the technicalities of conventional syllable division.
In the final part, I will take you on a whistle-stop tour of phonemes, and why one man's draw is another man's puddle.