Vowels, Diphthongs, & Semi-Vowels

Today I wanted to talk about vowels. More specifically, how vowel sounds combine. Combining vowel sounds creates diphthongs; thats the basic definition of a diphthong. But, there is another category, and its used a lot in English. Its called a "semi-vowel" and it includes letters like r, w, and y. This took me a second to wrap my head around, because I'd always thought of w and r, and, to a lesser extent, y, as consonants. But think about it- a consonant is a sound we make by impeding the flow of air through our mouths (p, m, s, b, z, even h, a little), but you're not really putting your tongue anywhere when you make an r. You could argue that you use your lips to shape a "w" sound, but when you sound it out, its pretty obvious that w is pretty much an "oo" sound combined with whatever is before or after it (row, water, coward). So my amateur-linguist definition of a semi-vowel is: a diphthong or vowel sound that is used as a consonant. This way, the semi-vowel can technically break some of your phonology rules for vowel combination, and make the conlang feel more natural and real.

So the conlang question is, do you want to make special rules about how vowels combine or don't combine in your language? No vowel combinations? Or every syllable must have at least one consonant and vowel? I originally built Pitak to not have any diphthongs or semi-vowels, but then I was looking at Tolkien's Sindarin language and Toki Pona, and realized I'd really like to have at least one semi-vowel in there, so I added w. As far as what languages seem to have which, I'd say that more primitive languages seem to have less combined vowel sounds, and established, evolved languages seem to have more diphthongs or semi-vowels mixed in. And further, think about whether the vowel combinations denote certain cases. Diphthongs or semi-vowels could show that a word is past tense, a command, or plural. Cool stuff.