Occasionally I read forum posts or blog posts or project pages on Ravelry complaining about the “hump” in the center of the top edge of a certain kind of crescent shawl. Some people speculate that it has to do with what kind of cast-on is used, but I think it’s inherent to the structure of the shawl itself (though sometimes it appears and other times it doesn’t).
Here’s why I think that.
This kind of shawl is knit by casting on a smallish number of stitches and then knitting flat, adding three stitches at each selvedge every two rows. If you were to cut off the selvedges and just lay the shawl flat without any blocking, this is more or less what you would see:
Note that the cast on edge is at the bottom row. This kind of shawl is actually a point-up triangle with all the increases at the edges, and the cast off edge being the long one. However, once this shawl is actually blocked to suit the knitter’s desires (with its selvedges mercifully not cut off), the two selvedges make a gentle curve that becomes the top of the shawl as it is worn, while the last row curves around at the bottom.
The selvedges tend to be on the tight end of things because of the structure of the shawl, so they naturally curve on their own, but it is the blocking that forces the final shape, as shown in these very rough sketches. How well this kind of knitting stretches depends on a variety of factors: what stitch pattern is used, how many rows are knit, the fiber content of the yarn. Also, I’ve seen some designers suggest using a larger needle for the wider rows of this shawl, which would help those rows stretch around the outer curve more easily.
Anyway, it is my thought that the hump is formed when the initial rows of the shawl are pushed away from the rest of the knitting in the blocking. If the outer rows don’t stretch enough (as shown in the left sketch just above), then if their selvedges are forced into a straight line, they will push the shortest rows into the bump.
Having the outer rows of the crescent be knit in a stitch pattern with more give should help with that, as should knitting them with a larger needle. (Or having the design add extra increases evenly across the rows somewhere in the latter part of the shawl.) But starting with different numbers of stitches or different kinds of cast-ons probably won’t make much difference. I haven’t done a lot of testing of this guess, mind you. It’s based on lifelong experience working with textiles.