This blog post is a work in progress; I'm collecting here the notes I've written on this over the years and will eventually polish it up into a real post. For now, it serves as a place to help crocheters choose the best yarns or threads for my crochet patterns. It's also a way to appreciate the key differences between what we call "yarn" vs "thread."
Cotton crochet thread of the same thickness as a yarn is often fundamentally different in its behavior when crocheted. One big reason (among others) for this is its direction of twist. Most crochet thread is "z-twisted" while most yarns are "s-twisted." (There aren't many exceptions!)
Here's an interesting post from the Caron yarn company explaining the twist direction of yarn.
I find that this factor makes the biggest difference for most crochet jewelry I design, and when I'm using very tall stitches because of all the yarn overs. Yarn overs either add to a yarn's twist, or subtract from it (i.e. unwind it).
I try to use triple trebles with z-twisted yarns because an s-twisted yarn, if it's not tightly plied, will come untwisted from all the yarn-overs and is unpleasant to work with and look at. Splitty yarn can really slow down working off so many loops with each stitch, and I want the stitches to end up looking good enough to be worth the effort, instead of stringy or unbalanced. (I think this matters more for right handed crocheters than left handed because of the kind of working twist we add/subtract, not sure.)