First, some terminology. I'm going to be discussing number of plies here. Ply is used in two main ways in the yarn world. The first has to do with the weight of the yarn; for example, in some parts of the world an 8-ply yarn refers to DK weight yarn, regardless of how many actual plies are contained in the yarn.
Instead, I'm going to be talking about the actual number of plies a yarn has. Take a look at some yarn that you have next to you. Hold about an inch of yarn between the thumb and index fingers of your hands. Untwist the yarn. Can you see the individual strands that make up the yarn? That's the number of plies in your yarn. Here's an example of a 2-ply yarn:
Yarn comes in various numbers of plies, from one (what is known as a singles yarn) through a much larger number; there are also different ply structures. But to keep it simple for right now, I'm going to be discussing a singles yarn (just one strand that is not twisted together with any other strand), a 2-ply yarn, and a 4-ply yarn. I'm also not going to take other factors (such as twist) into consideration, and all the yarns in the examples are 100% merino.
So if you're trying to decide on a yarn for a shawl with a lace stitch pattern, how many plies is the best for that shawl? Here's a picture to show you how the same stitch pattern on the same size needles looks across 3 different yarns:
The top teal swatch is in the singles yarn, the middle red swatch is the 2-ply yarn, and the bottom grey swatch is the 4-ply yarn. Which one shows off the lace stitch pattern the best?
The 2-ply middle red swatch most clearly shows those yarn overs and the lace pattern. The singles yarn at the top is decent, but the yarn overs are almost completely lost in the grey 4-ply yarn at the bottom. What you notice instead is the lines made by the decreases (k2tog and ssk). That's because a yarn with 3+ plies is more rounded with the plies nestling in towards one another and is thus better with a textured stitch pattern (so the decrease lines show better) whereas a 2-ply yarn is more flat with the two plies pushing against one another and is thus better for a lace stitch pattern (so the yarn overs show up better).
So the next time you plan to knit a lace shawl, take a look at the number of plies in your yarn. If it has 3+ plies, you might want to choose a different yarn, especially if you have a 2-ply yarn option. (The singles yarn is an option, but it'll show wear faster because it's more likely to pill and fuzz, so you'll want to keep that in mind as well.)
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