Yesterday evening I was casting on a helmet liner. I've made about a half-dozen of these over the last few years. In fact, I plan to be teaching a workshop on helmet liners at Rare Purls in Duluth. A helmet liner begins with a lot of 2x2 ribbing. Since the ribbing needs to stretch to go over the head, the cast on needs to be very stretchy. There are several ways to achieve this -- a long-tail cast-on with a double-thickness of yarn in the tail (Lucy Neatby), a German twisted cast-on (Beth Brown-Reinsel), or a tubular cast-on (Elizabeth Zimmermann). I got to thinking about the tubular cast-on.
Imagine the black line in the drawing is a row of knitting, all by itself without a row above or a row below. The bright green line represents a piece of waste yarn or a narrow needle or wire. If you've done Elizabeth Zimmermann's provisional cast-on, you've done this on your needles. You create a single row of knitting that is ready to be worked in either direction because that single row is folded in the middle. In a typical tubular cast-on, you'd continue with knit one, slip one for one round; then slip one, purl one for the second round. At that point you either go right into your knitting or you work those two rounds again if you want a little more edge. The cast-on is a little tube -- or a bigger tube if you work more rounds. Because is has the same structure as knitting, it will behave as knitting and stretch and give. Once the edge is established, the waste yarn is removed.
Now, there's a nifty little game you can play with this. Imagine that the black "yarn" is slightly stiff. Imagine I have it laid out as in the diagram, with a narrow piece of stiff green wire across it. Imagine I fold the lower stitches up. Counting from the right, the odd numbered stitches would lie behind the green wire and the even numbered stitches would lie on top of it. In other words, the odd stitches are purls and the even are knits. If I worked purl one, knit one across the row, the edge would be mysterious indeed. This is why Elizabeth Zimmermann's provisional cast-on can be so great for 1x1 ribbing.
But the helmet liner is 2x2 ribbing. Hmmmm. I decided to mix it up a bit by working the stitches out of order. When I needed to, I skipped the next stitch the get to the one I needed. The result is that every-other knit-purl pair is crossed. Here is the result:
It is a little dark with the black yarn, but you can see there is a large then small then large then small rhythm in the cast-on edge at bottom. I'm going to tear this out because I think this edge is too decorative for military use on a helmet liner. But it may have uses later in other projects.
Use Elizabeth Zimmermann's provisional cast-on with waste yarn.
Work knit two, purl two on the very first row. Treat the stitches in front of the waste yarn as knits. Treat the stitches in back of the waste yarn as purls. Work stitches out of order when you need to to get to your next knit or purl as needed. There will be times when you are skipping the nearest stitch on the left needle, working the next one in line, and then returning to the skipped stitch. It is this 1x1 stitch twist that produces the scallop.
After you get a couple rows established, remove the waste yarn. Smile.