Traditionally, certain shorthand names have been given to the categories of yarns. I've made a little chart to show you most of the ones I've seen or heard, and to demonstrate how I think of yarn classification in my head.
Like any set of guidelines, there are always exceptions, so consider this a way for you to conceptualize things. I've also added some anachronistic and international terms, which you may see when using a vintage or non-US pattern. If you should need to substitute a yarn, the starting point is this chart. Due to popular demand (I love it when you beg me), I'm going to talk about yarn substitution at length (or do I mean "ad nauseum"?) in another post, but finding where the specified yarn falls on this chart is the starting point to finding a good substitute.
Within the past few years, the Craft Yarn Council, or whatever the hell it's called, prepared its own "Standard Yarn Weight System." Their website grandly states:
The publishers, fiber, needle and hook manufacturers and yarn members of the Craft Yarn Council of America have worked together to set up a series of guidelines to bring uniformity to yarn, needle and hook labeling and to patterns, whether they appear in books, magazines, leaflets or on yarn labels. Our goal is to make it easier for consumers to select the right materials for a project and complete it successfully.
Here's what their chart looks like:
Now I will be the first to say that there is some valuable information in the chart. It lists number of knit stitches in stockinette stitch (over four inches); recommended needle size; number of crocheted stitches in single crochet; and recommended crochet hook size. But I still hate it. It's really counter-intuitive to me and for that reason, I'm unlikely to use it on a day-to-day basis.
First of all, there are some glaring omissions from the chart. Lumping all yarns that knit at seven stitches per inch or finer together into one category really isn't accurate. There are fingering weight yarns, then there are lace weight yarns which are much lighter, and there are even cobweb weight yarns which are, like the name suggests, very much like knitting with spider webs. To suggest that all of these are interchangeable, simply because they are really skinny, does these yarns a disservice and has the potential to be highly confusing. (You could make the same argument about the heaviest class of yarns; there are yarns that knit at 2 sts per inch and yarns that knit at 3 sts per inch, and if you were to make a sweater treating those yarns as interchangeable, you'd end up with two completely different-sized sweaters).
I also find the use of gauge over four inches (as opposed to stitches per one inch) to be user-unfriendly. I understand that you will get a more accurate gauge count if you measure over a larger swatch, but I always end up dividing the number by 4 to get a stitches per one inch number. That's how I think of them in my head: "5 sts per inch," not "20 sts over 4 inches." This particular gripe may be peculiar to me; if you're accustomed to thinking about gauge over 4 inches, you'll be just fine.
But my biggest complaint is how they have taken the interesting, quirky names for classes of yarns and substituted numbers. Numbers that don't have any identifying characteristics about them. Is Category 3 worsted weight? sport weight? who knows? who can remember? Is Category 1 the really fine yarns, because they are thin and one is the lowest number; or are they the really thick yarns, because the number of stitches per inch is so few that they put them at the low numbers? Is Category 5 worsted weight, since worsteds knit at 5 stitches per inch? Is Category 6 fingering weight, since "finger" has six letters in it?
I'd much rather talk about "DK" or "doubleknitting" yarns, knowing that this is a wartime term that meant the yarn could knit comfortably at either 5 or 5.5 sts per inch (double = 2 = 2 gauges, get it?). I'd much rather see a reference to a baby yarn, knowing that it refers to something thin and fine enough to put on a baby, visualizing Dale of Norway baby patterns that knit up at an ungodly 7 to 8 stitches per inch. I'd rather hear about a polar weight yarn, immediately getting the image of how thick and bulky a yarn would have to be in order to be useful in the Arctic regions.
Maybe I'm a traditionalist, or maybe it's just that you get so accustomed to a particular set of measurements or standards that your old brain just can't cope with translating everything into the new ones.
Don't get me started on "ply"
You'll notice some references to "ply" in my yarn chart. I've put them in mainly because some older patterns and some from other countries refer to yarns by their ply. Allow me to state, unequivocally and without hesitation, that ply has nothing to do with yarn weight or gauge. Do you hear me? Nothing. NOTHING, I TELL YOU.
Let's get basic. A ply is nothing more than a strand of fiber that is twisted around one or more other strands to make up a piece of yarn. I never really appreciated this distinction much until I started to spin, but if you pay attention, you can see that certain yarns, like Manos del Uruguay, are "singles" or single ply: the yarn consists of only one strand. When you start to spin, people will talk about your "singles," meaning the strands of yarn you produce and use by themselves, singly. Other yarns consist of multiple, very thin strands that are twisted around each other. At the cut end of your yarn, you can sometimes see the plies untwisting from around each other. If you think of something like Reynolds Odyssey, or some Noro yarns, in which the plies are different colors, you can see the plies twist around each other. One of the easiest ways to improve the look of a newbie's handspun yarn is to ply it: twist two strands of it around each other, and watch how magically the thin parts in one strand balance out the thick parts in the other, and vice versa.
If you think about it, there is some logic to the idea that the more plies, the more strands of yarn twisted around each other, the thicker the yarn you'll end up with. And there may have been a time, perhaps, when you could reliably tell a yarn's size and how many stitches per inch it would knit with based on the number of plies it had. That's why you'll see the term "4-ply" as an old synonym for fingering weight yarn, or the term "8-ply" for sportweight.
However, in the wonderful yarn world in which we live today, there is no longer any correlation between ply and yarn thickness or gauge. Technology is such that very thick yarns, like Manos, can be single ply, while infinitesimally thin multiple plies can make up a lightweight sock yarn. I've included the "ply" terminology in my chart because you might run into a pattern that uses these terms in describing a yarn, but rest assured that other than using them as a clue to the yarn's size (and if you've got a gauge measurement, that's a much more reliable indicator) you shouldn't worry your pretty little heads about them one bit.
Now it's off to kiss three little munchkins goodnight, and then maybe, just maybe, a few rows of knitting before I go comatose.