I invited Monica Ann McDonald, the fabulous spinner behind The Fine Lime to talk about YARN! I think a lot of people get confused about yarn weights and plies, so Monica being the sweetest person that she is, accepted my invitation to explain us what those things mean.
Big and Small and Not at All
A quick review of yarn weights and plies
by Monica Ann McDonald
What does the term weight refer to?
Weight, when we're talking yarn, is really talking circumference. How big around is this strand of yarn? There are a few kinds of labels we use to talk about weight.
NUMBERS - This is common on US and many international yarn lables. There's just a number, 0 - 6, that tells you how fat the yarn is. ) 0 is skinniest, 6 is fattest.
WORDS - I cannot possibly list them all, but I will list a whole bunch by weight. Thread, cobweb, lace, baby, fingering, super fine, sock, sport, fine, baby again (how's that for fun?!), DK, light, light worsted, worsted, medium, aran, afghan, fisherman, heavy worsted, bulky, chunky, extra bulky, craft, rug, roving. Yeah. Great. Super helpful.
STITCHES PER INCH - for most yarn lables, this means the knit stockinette stitch on the recommended needle size.
WPI - wraps per inch. It means the number of strands, placed side by side, that it takes to make one inch. This is the most useful for me. I've blogged about it here: http://thefinelime.blogspot.com/2014/05/what-wpi.html
YPP - yards of yarn per pound. Most commonly used in the textile industry for cloth manufacturing.
Plies - this is used for labeling in the UK and Australia. It doesn't actually refer to the number of plies (strands twisted together) that make up the yarn. It refers to the number of plies of a standard sized thread it typically takes to make that size of yarn. Yes, that is a bit confusing, but worth mentioning in case you buy a yarn manufactured in those countries.
The best chart I've seen to break all of this down is the Wikipedia one. http://en.wikipedia.org/wiki/Yarn_weight
A simple chart is available at Fluff and Fuzz
Why does it matter?
Well, here's the thing. Pattern designers are working hard to create patterns for you. They don't really have the time to go dashing to the LYS and making up the pattern in 13 different yarns at varying price points to give you a range of yarn choice options. So you go yarn shopping and you find this glorious yarn that you would like to use for the latest Just Be Happy pattern. But you can't use it because the pattern was written for one specific yarn brand. Right? Wrong.
You can find the WPI of the original yarn (for ultimate accuracy) or just the weight name or number, and compare it to the weight of the yarn you'd like to use. If its reasonably close, within a few wraps per inch or in the same number category, you will be able to swap yarns with little to no alteration in hook/needle size or number of stitches.
Wait, so what is a ply anyway? And WHY IS IT A THING?
Plies are just the sumber of strands twisted together to make a yarn. Sometimes its just a SINGLE PLY, like Knit Picks Chroma or a lot of handspun yarns. Many handspun yarns are two ply or three ply. You may find it helpful to know that in handspun, Andean ply is a 2 ply yarn and Navajo ply is a 3 ply yarn. Those are just names/origins of a certain plying method when they make the yarn. Most of your millspun yarns are 4 ply. Some are even more.
Why ply? Well, if you're using a mill machine to spin the yarn, it may only make one weight of yarn. So if you want the yarn fatter, you will need to spin those strands together.
Why does ply matter to a hooker or knitter? Plies make yarn just a little less soft and much more durable. You can think about plies when you are planning your project. If you are making something that is mostly to accessorize or just sit on the body, like a photogrpahy prop or cowl, a single ply yarn will be soft and lovely. If you are making something that is going to take some abuse, like long sleeves on a sweater, gloves, mittens, or socks, you will want to use plied yarn so it doesn't wear out as fast. More plies = more strength.
I hope all this information helps you to be adventerous in your craft and your yarn selections!