The sweater is knitted the round, using the Entrelac technique.

EDITS

1. page 3 for the back triangle at the neck. There are instructions for a left leaning triangle (like at the underarm), but it should be a right leaning triangle (follow instructions for left leaning triangle but all “k” should be “p” and vice versa).
2. The 4 triangles around the sleeves are the same as the left-leaning base triangles. The pattern says to leave one triangle free and not seamed to anything, but I just picked up along the one square without live stitches and then continued knitting triangles with the live stitches, so there’s nothing to seam at the end.
3. Right leaning triangles prior to cuffs (same type as underarm and at neck)