Small differences in ease, on the order of an inch or two can make large differences in the fit and comfort of a garment. Wearing a garment with positive ease when it was designed for negative ease can yield unflattering results. For example, if a person with a petite frame wants an oversized comfy sweater, she can't just buy an extra large size of a sweater that is designed to be form fitting. The shoulders would be too wide, and the waist shaping would fall in the wrong place on her body. If she wants an oversized sweater, she needs to find one that was designed to be oversized on her frame.
Ease is especially important when knitting fitted garments like socks. A sock with positive ease through the instep will slide around on the foot or bunch up and create the potential for blisters. A sock with positive ease through the calf will fall down and look silly.
Knee socks that stay up are a challenge for handknitters precisely because of the issue of ease. The rainbow socks in the picture above are my second attempt at knee socks, and the first successful ones.
The black and grey striped socks at the right are SmartWool knee socks, which do stay up but unfortunately are really too snug, which is part of what drove me to attempt the craziness of knitting my own.
My first pair of hand-knit knee socks is the purple and grey ones in the middle of the line-up above. The general guideline for handknitters is to knit socks with 10% negative ease, so the dimensions of the sock are 90% of the dimensions of the wearer's foot. When planning that first pair, I carried this 10% idea all the way up the calf, measuring the circumference of my leg at 1" intervals from ankle to knee, then multiplying each measurement by .90, then knitting to those dimension. When finished knitting, I pulled the socks on and stood up, ready to be proud, and was instead dismayed that they promptly fell down. They would only stay up if held in place by my tall dress boots.
I turned to the forums on Ravelry for research and learned that knee socks need a much greater degree of negative ease to stay up, but not nearly as much custom shaping as I had done. One knitter whose other suggestions about socks had been helpful, suggested at least 3" negative ease for knee socks. Figuring that for an average calf, three inches would be 20-25% negative ease, I decided to try a sock that was 9.5" at its fullest circumference when unstretched. (I should note that my calves are quite large. My feet are hollowed by high arches, my heels are narrow, and my ankles are fine, but my calves are muscular and wide, 16" in circumference at the fullest point.) The rainbow socks were a success! I eventually ripped the purple ones back to the ankle and reknit the calf to look more like the rainbow socks, and they stay up now, too.
The blue socks at the left of the lineup provide a lesson in too much negative ease. They are ribbing through the foot and the lower leg, so they are not nearly as tiny as they appear to be in this picture, and they fit my foot and lower leg. The top cuff, however, has cables, a knitting technique that involves crossing stitches over each other to create texture in the finished fabric. I fell in love with this pattern because of the cables. But while crossing stitches over each other creates fetching designs, it also reduces the elasticity of the knitted fabric, and these cables draw the fabric in so much that they will not go over my calves at all. The socks are in deep time out right now, while they contemplate this offensive behavior on the part of their stitches, and I decide how to modify the pattern.
Other knitters remark on my knee socks frequently, often because they notice that mine are staying up and their only experience has been as disappointing as my purple and grey ones. So here's how I do it:
I start socks at the toes. This allows the toes and instep to be a swatch, and I can correct any issues of gauge (8-9 sts/in is ideal for socks) and fabric density in this part of the sock before turning the heel, especially if I'm working with a yarn that's new to me.
To create room for the calf, I work paired increases at the back of the leg every other round for several rounds. Then, at the top of the calf, I work paired decreases to remove one-inch's worth of stitches before switching to smaller needles for the ribbing.
The whole reason I learned to knit was to make my own socks, and I'm happy with what I've accomplished so far, but I'm continuing to experiment with ease and pattern and shape. An unexpected side effect of this process has been a greater understanding of how garments interact with bodies generally. Ease is not a concept most of us think about when we shop for garments off the rack, but we should.