When most people think about verse, they tend to think of rhyme first and foremost. Understandable, since that’s the defining quality of so much poetry. But there’s a whole other side of the word to explore, a front-end counterpart to the back-end rhyme.
Alliteration
Alliteration is the repetition of a sound at the beginning of a word, a mirror image to rhyming. It’s not quite as obvious these days, as rhyme and rhythm have won our hearts and minds, but it has an illustrious history. Some of the earliest Anglo-Saxon verse was composed using alliteration, as were epics from around the Western world. Classics such as “The Raven” and “Rime of the Ancient Mariner” have sections of alliterative verse, as do children’s nursery rhymes. Peter Piper probably needed something to catch the spit from all those P sounds. And who can forget all those old cartoons with hilariously alliterative newspaper headlines? Those were a thing, and they still are in places.
Echoes of alliteration are all around us. Like rhyme, the reason borders on the psychological. In oration, the beginning of the word tends to be more forceful than the end, more evocative. So punctuating your point with purpose (see what I did there?) helps to get your message stuck in the minds of your listeners. They can “latch on” to the repetition. Wikipedia’s article on alliteration uses King’s “I Have a Dream” speech as an example: “not by the color of their skin but by the content of their character.” Notice how the hard K sounds beginning each of the “core” words grab your attention.
To be alliterative, you don’t have to use the same sound at the beginning of every word. The rules of English simply can’t accommodate that. (Newspapers cheated by removing extraneous words such as “a” and “the”.) It’s the content words that are most important, especially the adjectives and nouns. However, alliteration tends to be stricter than rhyme in what’s considered the “same” sound. Voicing differences change the quality of the sound, so they’re out. Clusters are in the same boat. On the other hand, sometimes an unstressed syllable (like un- or a-) can be ignored for the purposes of alliteration.
Assonance
Alliteration is concerned with consonant sounds. (I did it again!) Assonance is different; it’s all about the vowels. What’s more, it’s not limited to the beginnings of words. Rather, it’s a vowel sound repeated throughout a phrase or line of verse. Vowel rhyming can be considered a form of assonance, but it’s so much more than that.
Assonance pops up everywhere there are vowels, which means everywhere. It’s very well suited to small utterances, such as a single line of a song or a proverb. As with alliteration, it’s not an absolute requirement for all the vowels to be the same, but those that are need to be essentially identical. And it’s the content words that are most important. Schwas, ineffectual as they are, don’t even appear on the radar; a and the aren’t going to mess up assonance. But any other vowel is fair game, in English or whatever language you’re using.
In conlangs
Alliteration and assonance are perfectly usable in any context, and they can be made to fit any language. They might not be quite as permissive as rhyme, but they can have a greater lyrical effect when used properly. (And sparingly. Don’t overdo it.)
These literary devices work best in languages with patterns of stress. That stress can be fixed, but that narrows your options slightly. Inflectional languages with fixed final stress are probably the worst for alliteration, while initial stress gives the most “punch”. For assonance, it’s not so vital, but you want to make sure your vowels aren’t being forced to a fit a pattern.
Both alliteration and assonance are easiest to accomplish in languages with smaller phonemic inventories. That shouldn’t be surprising. It’s far less work to find two words that both begin with a P if your only other options are B, D, K, and S. With these smaller sound sets (are you kidding me?), you can even create more complex styles of alliterative verse. Imagine a CV-type language with interwoven alliteration patterns, where the first and third words of a line start with one sound, while the second and fourth begin with a different one.
The other end of the spectrum holds English and most European languages, and it’s less amenable. You need lots of words, or you’ll have to get some help from stress and syllabics. That’s how we can have alliterative English: by ignoring those tiny, unstressed prefixes that pop up everywhere. It’s possible to make it work, but you have to try harder. But trying is what this is all about.