From the pattern: 

"This fitted, lacy sweater was inspired by some mysterious mohair-blend yarn and the lacy Grecian plait stitch, found in Mary Thomas's Book of Knitting Patterns.

The mysterious yarn had lost its labels, but Classic Mohair seemed like a good substitute. The mohair counteracts the three-quarter sleeves and low neck, making it warm enough to wear sooner, rather than later, while still feeling springy.

The Grecian plait, which really does look like rows of tiny braids, is formed by purling wrong side rows with a needle about twice as large as that used for the right side. The resulting enlarged stitches are slipped over the end of the needle to form the braid."