This is my happy project face from back in February. I had just finished my first Custom Fit Sweater in my own Helix yarn, held double, and I was so happy! Now that cooler temperatures are back, I am wearing this vest again all the time, and loving it just as much.
Much of why I love this vest has to do with the doubled yarn: The color is perfectly even, it's cozy without being bulky, and it was a very fast knit.
If you're wondering why a sane knitter would choose to double the amount of yardage they're using on a project, there are a number of good reasons. Holding the yarns this way changes the resulting knitted fabric (for the better, in my opinion), blends colors, and speeds up your project. It's also a great way to test-drive a fine yarn with reduced commitment.
So let's say you have two swatches, both on size 7 needles. One swatch is in a round 4-ply worsted yarn, and the other is two strands of sport-weight held double. The sport-weight held double is going to have a springier feel to it and more structure and body.
This structured hand-feel is the result of the shape of the yarn. A cross-section of yarns held double is quite flat compared with a round yarn (like a 4-ply worsted). This flatness in the yarn gives a flatness in the fabric. It also makes the yarn fill in visually, which is why it's a great technique for softies and other stuffed toys where you don't want to stuffing to be visible through the knitted skin.
If you love using semi-solid yarns but hate pooling, or having to use two balls at once, holding yarn double may be for you. It helps spread out high and low contrast areas of the yarn, since highlights and lowlights tend to cancel each other out across the two strands. You end up with a more uniform overall look than when working with just one strand or from one ball.
If there's a fine yarn you've been thinking about selecting for a major project (like a sweater than will take you months and months), you may want to try a smaller project (like a vest!) with the yarn held double to see if you really like it and see how it wears.
Or, perhaps, you just want your hand-knit sweater without an enormous wait. MelSKi's gorgeous Ikaika sweater is designed with our Helix yarn held double, and it knits up fast - in addition to having amazing hand-feel.
Try a yarn-held-double swatch, and let me know what you think!